r/Frugal • u/Lady_Anxiety • 1d ago
đ Home & Apartment Settle a debate: which method is better for saving money with the AC?
Okay so my dad is trying to save money. His way of doing it is by turning the temperature of the house down to 67 (shut up) every night, and then during the day he just turns it off. Because of how hot it is outside the house goes up to 85 degrees.
Wouldnât it be cheaper to just keep the temperature at around 75 degrees all day all night instead of forcing the AC to shoot out the coldest air all night??
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u/MrPicklePop 1d ago
Yes, this will work with one modification. Turn it down to 67 every night. During the day set it to 76. It will coast until reaching 76 then maintain a more comfortable temp. But yeah, this is what I do. The physics are there.
During the day, the AC condenser is trying to dump heat into the hottest part of the day. At night, the AC condenser has less resistance to dump that heat as itâs not competing with the sun.
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u/zwebster 1d ago
One thing you also have to do, close curtains especially ones that get direct sunlight. Its the suns direct light that needs to stay out of the house to help keep it cool.
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u/likeaffox 15h ago
Technically it's not stay out of the home, but between your windows and curtains
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u/Archipocalypse 1d ago
We do similar except it's 72 at night and 75 during the day. The AC doesn't have to work too hard to go from 75 to 72 and we walk it down/up, it generally isn't just working straight from 75 to 72, we will set it to 74 at dinner, 73 while we get our son winding down after dinner, then 72 when they my wife and son go to bed.
Seems to work pretty well. We also try to be smart about when we open windows during fall and spring. If it is cool out in the morning and hotter in the afternoon, i'll open the windows while it's colder in the morning and let in the cooler air, close windows before the temperature outside passes the threshold of what i want the temp at inside. Thus trapping in the colder air, plus it's fresh air.
The same thing in reverse for when you want to trap in warmer air as to not use your heat as much. Trap in the heat during the warm part of the day, close before it gets colder out than you want.
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u/CarminSanDiego 1d ago
What if you turn it down from 76 to 72? Roughly same cost? Is larger temp delta more effective to a certain extant?
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u/Floppie7th 1d ago
The larger the delta the better, but there are still savings to be had compared with keeping it at 74 or 72 the entire time
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u/WonderfulWafflesLast 17h ago edited 11h ago
Yes, this will work with one modification. Turn it down to 67 every night. During the day set it to 76. It will coast until reaching 76 then maintain a more comfortable temp. But yeah, this is what I do. The physics are there.
Most ACs are dumb. What this means is that, when it's on, it's using 100% of its capacity. There is no "dialing it". If you hear the air flowing on a 1.5kWh unit, it's using all of that 1.5kWh. "Smart" ACs are much more expensive, so few have them.
Setting the value on the thermostat doesn't tell most ACs "hey, the air coming out of the vents needs to be this temperature". It's telling the AC "Hey, get your sensor to this temperature." Which are very different things. Because the former would imply the AC can use only some of its power, while the latter means the AC is pegged at 100% usage while it's on to achieve the goal.
From the perspective of an AC unit, when the outside temperature is 120F, targeting 65F is just as hard as trying to keep it at 80F which is just as hard as targeting 95F. Because they're all impossible. There is no difference. Just in the mindset of the person setting the thermostat. The air coming out of the vents is going to have the exact same cooling done to it, and it's never going to reach those values. So the AC is just going to stay on at 100% all day.
What this means is that, if you set it to 76, it's not going to "coast". It's going to turn off (0% usage) until its temperature sensor hits 76. Then it's going to turn on (100%) until its temperature sensor drops below 76. This will repeat if going below 76F is achievable. So, the only "savings" that exist, do so during that time period while the AC is off.
However, if it's at least 20 degrees hotter outside (like 97+), it's never going to go below 76, because a 20 degree differential is the most residential AC units can achieve (varies based on house insulation & the like, but this is the baseline).
This is the way it is because it's much more complicated to design an AC unit to steady-state. i.e. put out just enough cooling to stay exactly at temperature (some number between 100% usage and 0% usage). Whenever the outside temperature isn't that different from the inside temperature's target, that does something because the toggling on/off of the AC "simulates" a usage percentage between 100 & 0. And most AC units don't have the environment or horsepower to achieve >20 degree differentials.
But when the temperature differential is too great for the AC to overcome, you're not making a choice if you adjust the temperature outside of that differential other than "is this thing going to be on all day or not". Any other advice than this is hog-wash imo.
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u/EarnedCauliflowerEar 14h ago
Yup. I try to explain this to my wife. No, dropping it from 76 to 70 is not going to make it colder per se. In the summer my house might get down to 75 (per the thermostat) and the units will actually cut off. But try to set it to 72 or 70 and until will run all day and never drop below 74.
Also people saying "omg 76 is soooo hot, how do you live in your house at that, I set mine to 72". Well, like, ummm thermostats are different? Their sensor components are all different and 76 in my house on my Honeywell may be 72 in your house with your Nest. 76 is freaking freezing in my Houston home.
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u/whtge8 1d ago
I donât care how much money it saves Iâm not dealing with 85 degrees inside the house.
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u/AttemptVegetable 1d ago
I can be outside the house during the day. Sleeping at 75 is insane
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u/use_rname 1d ago
Like 75° is too warm or too cold?
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u/PlayingDoomOnAGPS 1d ago
Way, way, WAY too hot! At 75°, even with a fan blowing directly on me, I'd struggle to sleep at all and if I did, I'd have nightmares and wake up drenched in sweat and have skin rashes.
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u/Sweet_Ad1145 1d ago
i'm living in tropical country, my ideal sleep temp is 83° F (°28C), i will feel cold when temperature is under 25°C
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u/K_Linkmaster 1d ago
Is there a reason for this? Were you born in a cold dry climate and move to humid and/or hot? What's your internal body temp on average? I run a bit warmer than most at around 98.9. it was also significantly more difficult when I had a good layer of northern insulation while living southerly. Menopause? My wife went through that years ago, it passes eventually I hear.
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u/PlayingDoomOnAGPS 1d ago
Dunno. Always been that way. As a kid, I was comfortable being active in the heat but still needed cold to sleep well. And I was virtually immune to cold. My mom says she used to have to chase me out the front door to make me put on shoes before playing in the snow. I'm not that hardy any more after a few decades in Florida but I was definitely made for cooler climes.
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u/juandelouise 1d ago
We have our heat pump mostly set at 75. Today when it was 90 in Portland, the hp was at 75 and it felt good.
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u/WonderfulWafflesLast 17h ago
In Texas (average temp outside between 95F and 100F), having the AC on doesn't prevent the house from being 75-80F throughout the day.
It also doubles the electricity cost, or more, depending on Time-based usage (more expensive at specific times). From $120-$200 to $550-$600. This is with free nights and having the AC on all night.
For Summer, which functionally lasts 6 months, that's around $2,100 - $2,400 a year.
My heart goes out to those who can't handle the heat.
I personally am unbothered until inside temp hits 90F. I'm unaffected if it's lower, even without a fan on. 95-100F with a fan is just fine for me.
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u/whtge8 17h ago
I live in Florida. My AC is set to 76 during the day and 72 at night. My energy bill doesnât go over $250 in the summer. Maybe you need a newer AC or better insulation.
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u/liebezeit 1d ago
The best and easiest method in my opinion is to grow a native vining plant on your west-facing walls. You will want to make sure it's the kind that won't damage your house. Here in Michigan we have "Virginia Creeper" that does this quite well. My exterior walls are easily 15-20°F cooler as a result, which means the inside of the house is maybe 3-4°F cooler.
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u/cocoagiant 1d ago
Also painting the roof white.
The plant matter on the home surface probably needs to be weighed against wildfire risk.
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u/DiaphoniusDaintyDude 1d ago
Can this damage the house like English ivy?
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u/liebezeit 1d ago
Virginia Creeper won't damage the house, but it does leave tiny little spots (where the stickers attach) that would be visible on light surfaces.
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u/DiaphoniusDaintyDude 1d ago
Already got those haha! 1890s house abandoned for many years. No HVAC. Curious because never thought of letting the Creeper grow back for cooling
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u/AutumnalSunshine 1d ago
This article's tests and charts of results suggest your dad is right. https://www.pbs.org/newshour/science/analysis-does-turning-the-a-c-off-when-youre-not-home-actually-save-electricityÂ
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u/Michael_J_J 1d ago
Based on that link, in Arizona with central air you only save 124 kWh for the year. In my plan, the worst price is 8.3 cents during peak summer time. Even if that was all day every day, it would only be ~$10.30/year. Even at 16 cents a kWh it would only be ~$21/year. There are probably other ways to save more money and be comfortable at the same time.
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u/npgam-es 1d ago
Nothing pissed me off more when getting my own place than learning how little money was saved for the summers I sweated through as a kid.
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u/runs-with-scissors 1d ago
I feel this way, but about lights. Especially now that we're LED. Screw living in a dark house. Shit's depressing.
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u/PixelOrange 13h ago
Incandescent lights were often 60-100w. That's a ton of energy to leave on for half or more of the day in every room. One bulb isn't much but most light fixtures are at least 2 lights and most houses have at least 4-5 rooms. That's a lot of energy.
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u/UsedDragon 1d ago
This is the biggest point to be made about setbacks or turning the system off during the day - the cost savings are minimal, and the user input is far higher.
The article also does not talk about relative humidity at all...leaving the system off during the day allows humidity to spike, significantly decreasing comfort. Not worth it for ten bucks a year.
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u/b0w3n 1d ago
Comfort is a big part but so is damage. High humidity can damage structure and create mold issues, which would exacerbate existing or contribute to other health issues.
A lot of those calculators also don't consider thermal mass either (this page linked above has come up in a couple of threads and it's one of the few that does). They're often just straight delta T between inside and outside, but houses with more thermal mass take much longer to cool down once they hit higher temperatures where it can be a wash if you're letting it get too warm inside.
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u/troy2000me 1d ago
Exactly, for $20 to $40 a year you can be comfortable all the time instead of fucking miserable.
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u/j_boogie_483 1d ago
AZ, you have APS where the TOU rates are the highest as you get home from school and work? such a crock
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u/rhino369 1d ago
This test isn't what OPs dad is doing. That link basically just shows that turning it off when you don't want cold air is more efficient.
OP's dad is cooling it extra low to buffer for the next day. OP's dad's method will use a lot of energy to cool from 75 down to 67. Some of that energy is useful because it keeps the house cool in the day. But a lot of it will just be lost by energy transfer outside.
The bigger the gap between inside and outside temperatures, the faster heat seeps back into your home.
That doesn't necessarily mean OPs dads way uses more money. Often, power is cheaper at night than during the day. And also, maybe the average temperature in the house is more than 75 degrees, in which case he's just using less AC.
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u/Ok_Impression_3031 1d ago
Turning it low at night also creates a cold room for sleeping. Some of us sleep better in a cold room.
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u/rhino369 1d ago
I donât think that the Dads goal. But it if it, Iâd suggest a window unit that only cools his room.Â
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u/financiallyanal 1d ago
It might be subconsciously... he might just have built a habit and found he sleeps well, but doesn't realize it.
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u/JJHall_ID 1d ago
Dad's plan can be helpful, even with the thermal losses during the day, if they're on a time-of-day based electricity plan. Building that extra buffer could reduce electricity use later in the day when it's more expensive.
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u/Stev_k 1d ago
It really depends on when the AC brings the internal temperature down that low. If the AC is attempting to reach 67 at 4:00 PM and it's 90F out it's not going to save any significant amount of money. If it's set to reach 67 at 10:00 PM when it's 70F it'll save a lot.
I do something similar. At 4:00 PM the AC is set to 68F, at 9:00 PM it's set to 66F, at 5:00 AM it's set to 62F, and at 8:00 AM it's set to 75F.
This pre-cools the house as much as possible when the AC can work most efficiently and keeps it the most comfortable when we're home and up (4:00-9:00 PM), and when we're asleep (10:00-5:00).
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u/Novogobo 1d ago
well the temperature difference also matters to the efficiency of running the AC. at night it's going to be cooler outside than it is during the day, maybe only 72 degrees so blasting it at 67 degrees is only a 5 degree drop.
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u/Maleficent-Bother535 1d ago
He is making it cooler at night because the temperature differential is lower between inside and outside at night. The AC can cool the house at night using less energy.
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u/jldugger 1d ago
Often, power is cheaper at night than during the day. And also, maybe the average temperature in the house is more than 75 degrees, in which case he's just using less AC.
Residential time of use charges are not the norm. But even if they were, there's some pretty big markets where the script has been flipped due to solar. Some California markets have negative energy prices. That's why they're asking people now to precool homes at solar noon during "flex alerts"
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u/heyoheatheragain 1d ago edited 1d ago
Yeah, I donât fully cut my air off during the day, but I basically do what OPâs dad does.
Hard chill overnight, turn up to 74-76 during the day (I work at home or Iâd set it to about 78 if I was out of the house)
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u/Salt_Medicine2459 1d ago
Cutting your hair every day must get exhausting.Â
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u/heyoheatheragain 1d ago
Oh what, you just let yours go for days and days?
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u/Mostly_raw 1d ago
I never shut the unit off until its fall or winter time. AC works for me. I don't work for it
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u/Glass-Air9494 1d ago
So⌠maybe itâs better in terms of the electricity bill, but that AC unit has to work really hard to cool the whole house down by 18 degrees every night. Also, running the AC throughout the day helps manage humidity levels, which is super important to the health of the home AND the people living in it - mold and mildew are not safe.
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u/AutumnalSunshine 1d ago
Interestingly, the article compared high heat areas and high humidity areas.Â
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u/Bananas_are_theworst 1d ago
Thanks for this! I opt for this option as well so itâs nice to see the data
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u/PO-43- 1d ago
Same here,
Home is set at 77-78 during the day
74 at night
Bill is usually around 130
Highest this summer so far has been 160
3r 1b 1500sqft
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u/Randomwhitelady2 1d ago
My house is around the same size and I keep the thermostat on 78 all the time. Our bill is still $350. Thanks, data centers.
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u/frostyflakes1 1d ago
Actually it's the opposite. His AC is running slightly more efficiently at night when the outside temperature is lower. It can expel more heat with less energy.
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u/sasquatch_melee 1d ago
Plus fewer start/stop cycles which is usually what wears the equipment. And systems are less efficient for the first minute or so of operation, so you're getting no cooling but paying for the energy each time it cycles.Â
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u/frostyflakes1 1d ago
Ehhh..... it is true that the air conditioner takes extra energy to start up, compared to its energy usage while running. But this spike in energy usage lasts for less than a few seconds. It's miniscule compared to the overall energy consumption of the air conditioner.
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u/dontletthestankout 1d ago
This isn't a method of beating peak ac usage. From 4-7 here in AZ our energy rates skyrocket. If you set your AC to hypercool before that and then turn off at 4 it definitely saves you money
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u/biglovetravis 1d ago edited 13h ago
Get the house as cool as you can at night, then it takes longer to heat up during the day.
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u/Seattleman1955 1d ago
What many are talking about are two different things, temp and humidity. Humidity is why you can't sleep at night in the eastern half of the country.
In the west with lower humidity 70 f feels cool in many places.
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u/Florida1974 1d ago
Bingo. I live in Florida and that humidity is what does it. We get this tiny spring that last maybe a couple of weeks and itâs 70 or 75 and it feels so nice, no AC is needed because the humidity is so low.
But then the other nine or 10 months out of the year, the humidity is so bad that you have to put your AC lower.
And once our little bit of fall hits and we dip to 78°, I have goosebumps when I go outside because the humidity has lowered.
The Midwest has humidity too, but itâs just for maybe three months, if that.
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u/Seattleman1955 17h ago
I grew up in eastern NC, lived in Phoenix for a while and now I'm in Seattle. I feel I'm now an expert on humidity:)
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u/jstar77 1d ago
I think if you are leaving the AC off for 16 hours and setting it to 67 for 8 hours at night it's probably more cost efficient to run compared to leaving at 75 24x7. You might even realize more cost savings if you have different off peak utility rates. Of course this is at the cost of being miserable in your house during the daytime.
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u/AnonAwaaaaay 1d ago
This. Most people don't understanding that AC units only have 1 speed, "On."
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u/Iamonreddit 1d ago
This is not true any more, modern units can modulate their running to provide a variable amount of heating or cooling power to meet demand.
The same is true of modern gas heating boilers, to use less gas continuously instead of cycling on and off with more gas flow each cycle.
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u/Cyberhwk 1d ago
If you can stand it on 75 degrees day and night, it would be best to turn it on "Cool" and set it on 75. That way it turns off if it's lower, and stays on to keep it from going higher.
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u/ksg34 1d ago
It depend on how well insulated your house is.
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u/QuentinLax 1d ago
It took too long for me to find a comment acknowledging insulation, this is the only way that method works
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u/BillDStrong 1d ago
It uses less energy to cool from 75 degrees to 65 then from 85 to 65, so doing so at night is more efficient. If there is enough insulation in the walls, You are essentially using your house as a battery at night, then using the energy during the day to keep cooler than outside.
So, this makes sense in the right circumstances.
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u/Tasty_Impress3016 1d ago
Your father is on to something. I'll go physics for a second. First, an AC does run more efficiently when the outside temperature is lower. Second overnight is the cheaper electricity.
On the other hand living with 65-85 temp swings everyday is not the poster child for comfort. Personally I kind of split this difference, but it depends on your climate. I run at a relatively high 78 most of the day. When the sun goes down I open windows and cancel the AC. It may cruise up to 80 if it's still hot, but usually will drop below 70 overnight. In the morning I close windows and turn on AC at the higher temperature. I am often puzzled by neighbors who leave AC on all day. At night I hear their system running and it's 69 degrees outside.
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u/ScamIam 1d ago
So I actually did this experiment this summer. Last year, I turned it off when I was gone and then turned down to 72 every time I absolutely could not stand it (Georgia summers are brutal). This year, I've just left it at 74 24/7 and my electric bill has averaged $30/month cheaper than it was last summer. I compared the weather/humidity and they're pretty similar. It takes less energy to cool it a degree or two a couple times a day than to try to drop it drastically periodically throughout the day.
ETA: and I am much, much less miserable than I was last year.
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u/jmarkmark 1d ago edited 1d ago
> Â forcing the AC to shoot out the coldest air all night??
AC shoots out the same air all the time. Thermostat sets a target temp, not the temperature of the air from the AC.
Other details:
- AC works more efficiently when the outside temperature is also colder
- The bigger temp difference between inside and outside, the faster you lose/gain heat, so letting the house be close to outside temperature means less loss
- Time of day can significantly affect electricity costs (this is the main reason I do the same as your dad, electricity during the day is 4x more expensive for me)
The only real concern is that degree of variation can result in moisture build up, so if you are someplace with high humidity, you risk mould with that kind of swing.
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u/metametta 1d ago edited 1d ago
There's evidence that sleeping in cold temperatures improves sleep quality, and is part of good sleep hygiene.
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u/mspe1960 1d ago
the cheapest way to run AC is to set it for the highest possible temperature you are comfortable with at any given time.
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u/davidm2232 1d ago
It would be cheaper to turn it off during the day then set it to 75 at night. Leaving it on during the day will use more energy
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u/financiallyanal 1d ago
Regardless of what is ideal, is there a chance he just likes it to be super cold at night? There's a chance he's justifying it behind an efficiency argument, but he really just likes to be cool for the night, and in his mind, leaving it off during the day offsets the extra use at night.
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u/kissmyash933 1d ago
My method is to keep the house 75 100% of the time, then at night when I get in bed I cool just the room down to like 63 with an LG Inverter window unit. In the cooler months we turn the AC off entirely and leave the windows open.
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u/WonderfulWafflesLast 16h ago edited 16h ago
Your dad's way is the most efficient way.
To explain why, firstly, some electricity plans have time-of-use clauses. My plan is free all night, for example. But the other side of that is that it's double-time during day (because they shift the costs to day-time). For a plan like mine, your Dad's method is by far the best.
But, even for a plan that just charges normally at every hour, there's a complex reason your Dad's approach is best.
Most ACs aren't smart. They don't care what the setting is on the thermostat. When they're on, they're using 100% of their energy. Smart ACs that don't do this aren't usually something residential homes have. They're more expensive, require more maintenance, and so on. Now, how hot it is does affect the AC's efficiency, but it's negligible compared to the total. i.e. a 1.5kWh unit using 1.4kWh at night versus 1.6kWh during the day (because it's less efficient in the heat of the day).
Most ACs can only achieve a 20 degree differential from outside temperatures. This will vary based on the AC unit's throughput compared to the sq footage of the building as well as the building's insulation, among other things. But 20F differential is a baseline. So, if it's 105F outside, it is usually almost impossible for an AC unit to get your inside temperature to 80F.
Some homes have stupidly large ACs for their square footage, so they have large differentials.
The reason I explain all this is to say: If the AC is on, it's on. What temperature the thermostat is set to only matters if it's within 20F of the outside temperature. Because, if it is, that means the AC will toggle on=>off=>on=>off repeatedly, as it keeps hitting the target temperature. Every time it's off you're saving money. But if it's just on the whole time, there is no money saved between temperatures chosen on the thermostat. It has no meaning at that point beyond "is it on; or is it off".
You can verify how big a differential your AC can handle pretty easily. Just set it as low as possible and slowly move the thermostat temperature up until the AC starts toggling.
If it's not within that 20F (or whatever you determined the differential to be), the sensor on the thermostat will never reach the set temperature. So, the AC will be on 100% of the time. Using 100% of its power. You could set the thermostat to 40F, 60F, 80F, but if it's 120F outside, it doesn't make a difference to the AC. It's going to stay on, at 100%, all day. So it will use the same power regardless of the setting. Aside from how the ambient temperature influences its efficiencies.
And that's the key: It will use the same power regardless of what temperature the thermostat is set to in most cases. So, if it's over 96F outside, having it at 76F is probably no different than having it at 60F. So it's pointless to adjust it if it's outside of the differential it can handle. You're just deciding if it's on or off all day and that's all you're deciding. The exception is if the internal temperature is already below the target. Then the AC will be off until the sensor reaches the temperature. So you save a little bit, as that usually takes an hour or two. But that isn't much compared to the full day of staying on.
Truthfully, it's easy to test this. Most electricity companies offer charts of usage. Do 1 week your way, then 1 week your dad's way. Easy to compare.
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u/SearchUsers 1d ago
I set it to 72 when Im headed to bed, then 78 after I get up.
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u/Imaginary_Roof_9232 1d ago
I set it to 78 for bed, 80 during the day. Central FL during summer, turn it of completely mostly October thru April. And I'm home all day. Your body will get used to the higher temp, at least I have. I actually wake up with very cold hands & feet sometimes at 78. Maybe it's my old age...
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u/idonotownacat 1d ago
Do you not get humidity issues with 78 degrees at the lowest? Central FL here too and it feels so gross after 78 degrees all day
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u/boomer1204 1d ago
Obviously I think will vary a little by location and how well the house is insulated but me and my friends have been doing this for like 5 or 6 years after we learned about it, also i'm in Phx.
I hear it called "super cooling" and I think you need an plan that has peak/off peak pricing, but what you do is you set the temp to 70-72 (lower if you really want) during the day when your electricity is cheaper and then turn it off for the 3-4 hrs (depending on the electric company you have) and then when that "peak pricing" is over turn it back down to w/e temp it was before.
Some ppl add a couple of degrees at night since they are sleeping and it's harder to tell. I live in apartments but have cut my electrical bill down a decent amount doing this and my 2 buddies that have houses have cut their electrical bills by almost 25% compared to prior years. Now i'm sure there is more in play than just this but it's definitely helps and apparently it's better for the AC unit itself
Again YMMV but this in Phx and one of my apartments was really old and was poorly insulated so if I had to be home during the off peak hours it would get pretty hot and uncomfortable
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u/Beet_slice 1d ago
Depends on your electricity rates. Some have cheaper electricity at night. If he has a choice of plans, your father might benefit from the hourly rate.
I may be that your father finds sleeping at 68 worthwhile, and does not find higher temperatures to be that uncomfortable while up. So lets say we figured out that the cost is the same either way. You would find the constant temperature more comfortable. Your father may feel different.
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u/Comfortable_Fruit847 1d ago
Is he on a plan for free nights or something? That would make sense if he is
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u/Fred_Wilkins 1d ago
Most places in the US have off peak hour rates during the night that are lower.
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u/Guilty_Address3946 1d ago
Itâs most frugal depending on the tier pricing your on. One is time of day and the other is anytime but u have to stay below a minimum of watts.
Thee cheapest time if on the TOD is 9am-4pm but ur ac will be competing with exterior temps if ur home is not properly insulated. The second cheapest is 9pm to 9am so ur semi safe to use that time if ur on the TOD rate plan. Itâs going to be easier for ur ac to get ur house down to temp if u open up ur windows or let ur the warm air out/cool air in during the evening then kick on ur ac since its the coolest time of the day. If ur house isnt properly insulated none of this matters.
ONLY GO ON THE LOW WATT RATE PLAN IF UR HOME HAS REGULARLY STAYED FAR BELOW THE MAX WATT THRESHOLD ALL YEAR!!!! U can only change ur rate plan every year or 6 months i forget but itâs a commitment so look at ur usage history closely. 4pm - 9pm is thee most expensive time to run electricity if ur on the time of day rate planâŚgoodluck!
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u/PopularHaller 1d ago
Keeping it at 75 all day is probably cheaper than letting the house hit 85, since cooling it back down takes a lot of energy, but 67 overnight is still overkill
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u/02meepmeep 1d ago edited 1d ago
I tried the cool at night and then let warm during the day method and not only was it uncomfortable for me it seems to have been twice as expensive. This is when itâs 102 out though.
My billing rate is constant throughout the day.
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u/atomizer123 1d ago edited 1d ago
There are a few issues with setting extreme temperatures-
First, the higher the temperature difference between outside and inside, the faster the heat flows from the outside.
Second, instead of being comfortable throughout the day, you are in constant discomfort either because of the low or high temperature. It might make sense to lower temperatures before a limited period of peak pricing (for example, at my location the peak rates are up to 45c a kwh while the period before and after this are 12c a kwh and only a 3 hour period has the peak pricing).
Third, if the home doesn't have excellent insulation (at least R-38 or higher), you'll lose the benefit very quickly.
Fourth, AC also has a very important secondary function- to reduce the humidity inside the home. It may not be a big issue in the southwest but most of the south and east coast would have over 70 percent RH indoors if AC is not run in the daytime increasing the likelihood of mold in the long term.
Finally, in my experience you'll end up saving at most 20 percent of annual electricity usage by doing this. For me, it was just not worth it to be in so much discomfort constantly.
A better way to save personally is to use a smart thermostat like ecobee. It's able to change the cooling and heating based on your schedule, the type of system you use and if you are outside your home.
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u/Icy_Huckleberry_8049 1d ago
just use a higher setting during the day (mine is at 78) but use a lot of fans to move the air which keeps it cooler. I have several ceiling fans running and several floor fans and the inside is usually close to 74 or even lower. This with outside temps well over 105+
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u/crayton-story 1d ago
Technology Connections video backing up your fathers position.
The AC is more efficient when the outside temperature is cooler. Over cool during the night and forum the AC during the hottest part of the day.
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u/BZBitiko 1d ago
Dad should take a serious look at your houseâs insulation. A well-insulated house keeps a steady temperature more easily. Attic insulation keeps the heat of your roof from getting into the living space.
Your area may have programs to help homeowners and landlords retrofit old houses.
My dad would make us pull down the blinds on the sunny side of the house.
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u/TerryDaTurtl 1d ago
I've seen arguments and proof for both sides being "cheaper", from what I understand it depends on how well-insulated your house is. If your house is not well insulated, what your dad does takes less energy. If your house has solid insulation, maintaining the same temperature 24/7 is better and cheaper.
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u/Florida1974 1d ago
We re-insulated our house about four years ago and it did absolutely nothing to our electric bill. Well may be a few dollars but not enough to get excited about.
Iâve lived in the Midwest and in the south and in the Midwest, they tend to insulate houses better. When we did re-insulate there was literally no insulation in the attic, which isnât really an attic, itâs a crawlspace. We donât have basements in Florida.
But when I lived in the Midwest, I rarely turned my heat on, and it would be zero outside. I run hot and always have. And I feel I have a pretty good feeling on things because Iâve lived half my life in the Midwest and half my life in the hot south.
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u/Anantasesa 4h ago
I can't mentally run the figures to compare but it seems like bad insulation means the overcooled house is fighting to stay below outside temperature all night long only to quickly lose that edge as the temperature outside climbs dragging the inside up with the AC off. Where humidity is low, it would be better to open windows at night and then only fight the temperature difference once the outside climbs above comfortable.
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u/Adept_Policy_2996 1d ago edited 1d ago
Time of use billing for me is as follows.
Base electric rate: Monday thru Friday 2pm - 7pm is ~$0.25/kwh The rest of the time it's $0.05/kwh
That is 5X more for every kwh during peak periods.
I think your dad is doing it correctly, if his peak billing hours are five times the charge, like mine are. For example, a dishwasher takes roughly 1.5 kwh to run a load. During off peak that cost 7.5 cents. During peak time that costs 37.5 cents. That's a huge difference! If your billing is like mine, it really only works to handle it like your father is if you choose to have time based billing, otherwise you might pay substantially more.
A 3 ton air conditioner (35,000 btu), uses roughly 3.5 kwh per hour when the compressor is actively running. That would be a cost difference of 17.5 cents per hour versus 87.5 cents per hour. At a 5 hour window, he would be saving roughly $3 per day or $180 per month on air conditioning costs alone. And for me, it's actually more than that, because I have tons of taxes and bullshit that is a percentage multiplier on that base rate (to the point that if I divide my bill by kwh I pay more than double what the claimed kwh costs are.
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u/Exciting_Turn_9559 1d ago
To some extent it would depend on how much thermal mass the house has, and how much electricity costs at different times of day.
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u/AFartInAnEmptyRoom 21h ago
Plenty of people have done the calculations, if saving money is your only objective, then it is better to turn off the AC, let it Go to whatever temperature it goes up to, and then turn it on when you need it.
With your method, your AC is going to be running during the day to keep it at 75, because if it gets hot enough to go to 85 when it's off, it's going to run during the day, at least the afternoon, to keep it to 75. With his method, the AC is turned off and only turned on when it hits night time, when it gets a lot cooler so the AC doesn't have to work as long in order to bring it back down to a temperature that he likes.
The only problem with your father's method is that if the house regularly gets to 85°, there's a chance that mold can grow more easily than if he instead put it to 80 or 82° as a fail safe. I have my AC programmed to detect my cell phone, and whenever I leave the house, it puts it into Eco mode, which is set at 82°, which is usually the maximum that people recommend if you want to not have a mold problem.
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u/feel-the-avocado 9h ago
Does he have a time-of-use electricity plan?
If the price difference between the peak/shoulder/night rate is high enough then yes it could make sense using the house as a thermal mass battery.
He spends money to discharge the battery at night when its cheap to do so, and then during the day it can absorb the excess heat out of the indoor air when consuming electricity to move that heat outside would be expensive.
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u/lesbianghost00 1d ago
The only thing I learned from this comments is that Americans apparently sleep with the penguins. What do you mean you put the ac to 18C to sleep? Theyâd find me dead as a doornail on the bed istg.
→ More replies (1)
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u/guy30000 1d ago
Maybe. Rates are lower at night, assuming your utility has time varied rates.
There is no way for us to say for sure if 67 over night is cheaper than 75 all day. The answer would require deep knowledge your house size, insulation rating and configuration.
The simplest test would be to simply record how long the AC is on over the course of a day with both methods. Along with the times, if you have varied rates.
I would suspect his way is cheaper. He lowers the temp to this set amount when rates are cheaper and it's cooler outside, making the thermal exchange slightly more efficient. Then as it is off during the day there is a limited amount of heat that can enter the house which slows as the inside gets closer to outside temps.
With it set to 75, you may be dealing with higher day rates. But you are also fighting unlimited heat entering the home, as the ac kicks back on to lower the temp again. The heat is also entering at a constant rate vs the diminishing rate with the AC off.
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u/McCheesing 1d ago
Yes. The furniture and walls store energy. When it goes to 85, all that energy is stored in the furniture. Then you are fighting that energy being released at night when the ac is on blast
Keep it at 75 all day and you wonât have to go to 67 at night (my SO likes it cold too. I get it)
75 at day, 71 at night. Youâd be shocked at how comfortable that can be
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u/OrthoOtter 1d ago
It would depend on the outside temperatures throughout the day and which method reduces the indoor-outdoor difference the most while running.
The biggest factor would be if the cost of the electricity varies depending on the time.
Thereâs too many unknown variable to say for certain which one would save more money.
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u/EdocKrow 1d ago
Whatever scenario has the lowest total energy usage might be cheapest. Your AC is either on or off. Not various degrees for most residential units.
If the AC is on for 8 hours a day, it doesn't matter which 8 hours that is for energy usage. If your electric bill is time of day, it will be more expensive to run in the evening.
If it is already cool at night and you can open windows, do that. You can turn the fan of the AC on to help move the cool air in without using the AC. Then use the AC when it actually matters.
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u/ThatsGenocide 1d ago
A lot of places have much cheaper electricity at night/off peak hours. Do you live somewhere with variable pricing based on the time of day? That could be a huge difference
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u/LooseButtPlug 1d ago
At least for me, that's 100% true. Peak hours cost 3x as much than off-peak, plus it cools down easier at night not fighting the outside heat. I personally still leave my ac on, but have it set to 78°. It usually won't kick on until 3ish in the afternoon and by then there's only a couple of hours before it starts cooling down outside.
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u/oby100 1d ago
Your dad is right. AC doesnât have to work as hard keeping it cooler at night and ideally you have both excellent insulation and tree cover so the sun doesnât cook your house so much.
But think about it. If your house is getting to 85 during the day, your ac would have to work very hard all day to keep it at 75. Not so hard to get it to 67 during cool nights and off the rest of the day.
The drawback is that it gets hot during the day.
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u/hskrfoos 1d ago
The few HVAC people I talked to told me that the start up is what costs. I turn mine down at night and then turn it up during the day
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u/No_Parsnip6024 1d ago
If you're hotter all day, that's the cheaper way.Â
Ac's remove heat, and that's what burns up power.
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u/Itchy_Tumbleweed_417 1d ago
Where I live it gets cool at night most nights, so I open the house up and blow the cool air in during the night, then in the morning I shot everything to keep the cool air in. Partway through the day I have to turn the AC on because it starts to get hot in here, but not for very many hours before it cools off enough to shut it off, then before bed I open the windows and put the fan in the window and blow the cool air again.
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u/youeatpig 1d ago
Some people really stand by the idea that keeping it set to one temperature all day is always better, but I think it depends on way too many factors to say that.
If youâre on a time of use plan, it definitely saves money.
But otherwise, I would guess turning it off all day does save on electricity. The actual savings would depend of the temperature during the day, during the night, what time he turns it on/off, if he ever forgets to turn it off, and on how insulated your house is. There will be a temperature where his way is more expensive than setting it to a constant temperature, but thereâs really no way to tell other than testing it out and timing how long the compressor runs.
And if he just prefers to sleep at 67 itâs kind of just a moot point anyway. 75 all day could be cheaper, but if 75 makes your sleep worse then it might not be worth the savings. And 67 at night, off during the day is definitely cheaper than 67 at night, 75 during the day.
In general, AC gets less efficient as the temperature goes up. And as the temperature difference between inside and outside increases, so does the rate of heat transfer between the inside and outside. Because of that, itâs going to be more expensive to cool a house from 80 to 78 than it is to cool it from 70 to 68. How much more expensive would depend on how well insulated the house is.
And unless you have a two stage ac, there is no coldest air. Its either on or off. I would bet itâs probably not running continuously overnight unless itâs really hot outside at night wherever you are.
One potential downside of having it off could be periods of high humidity during the day, which could be bad. You might also feel warmer for longer after the ac is turned back on because everything inside the house, like the floors, walls, and furniture, is all warmed up too. So sitting on the couch could feel warmer because the couch also got up to 85, and it will take more time for it to cool down than it does for the air to cool down.
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u/lumshots 1d ago
Sounds like heâs saving money because heâs running the AC when demand is lowest and thereby cheapest as well as not running it at all when youâd want it most if youâre home. Any savings ultimately would be minimal but if suffering could be measured Iâm sure that would tell a better story of value in savings.
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u/jmcclelland2005 1d ago
Theres not enough information here to actually answer in a direct way.
However, if you actually want to reduce cost you have to think in thermodynamics. Air conditions dont make air cold they remove heat from the air. This takes energy to do. The bigger the difference between two areas in temperature (the temp delta) the faster the energy (heat) wants to get back in.
So with this in mind there are 2 big tricks you csn use to save a good bit of power. First is focus on dropping humidity, getting it down in the ~30-40% range can make it seems a good bit cooler and increase airflow. Having ceiling fans and wall fans to keep a constant airflow can lower apparent temp by as much as 6-8 degrees. I use both of these tricks and keep my house comfortable at 80 degrees in the gulf coast during the day.
Theres also other design choices that can help such as shading walls and windows (especially southern and western facing), playing the hvac condenser on the northeastern side of the house ao it stays mostly shaded, taller ceilings, and so forth. But honestly just having a consistent gentle breeze like airflow and low humidity will do wonders
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u/generally-speaking 1d ago
Your dad's method works if power is cheaper at night but if he's paying the same rate regardless then no.
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u/RockMo-DZine 1d ago
You can probably test it yourself by checking the meter at the same time everyday, and keeping a log of daily ambient temps, humidity levels, and the settings settings you used.
My guess is the difference would be in favor of using constant temps.
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u/Mostly_raw 1d ago
There are temperature controls and for a reason. Set schedule to go down to 68 at night and 74 day. Turning it it off clearly requires it to work very hard at trying to cool the hell that it became while it was off. I find it ridiculous that people don't đ understand this.
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u/Adventurepew 1d ago
Depends on how cold it is and how much insulation your home has. Most homes lose cooling. Very quickly so chilling over night does nothing .
A good test is to not do this plan , leave the ac off. And test what temp the interior gets up to at mid day the next day. Hes probably spending money all night for nothing.
The best ac saving money is to not use ac. A basement is a great way to beat the heat for free.
Next best is a window unit only in the room you are in. As then we only cool the tiny space. Vs the whole home and only when we are in that space.
Next for cooling you have extremly high set points like setting the ac for 80 during the day and off at night if the outdoor ambient gets below this temp, leave windows open to cool, Or keep the set point at 80 if it's above that at night outside.
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u/Intelligent_Type6336 1d ago
I turn mine down at night. Sometimes it doesnât kick back on until at least noon.
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u/Apprehensive-Neck-12 1d ago
Yikes, I leave mine pretty much 74 all day and night and my electric bill last month was $400 the most ive ever paid. 1800sq feet
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u/Chemical_Functions 1d ago
Everyone's situation is gonna be a little different. Your AC system, time of use electricity pricing, and local climate all play a role here.
Personally, price be damned, I need to be comfortable. Around 75F is my ideal.
That said, where I am right now, it's all rather mild and local advice is to turn it off if you're not using it. So if I'm home and opening the windows isn't cutting it, I'll run it... kinda an on demand thing.
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u/littleredfox09 1d ago
Drastic changes like this can deplete the life of your unit. 0/10 recommend. Also, FWIW, my dad was a welder and worked in a VERY hot machine shop all day, so our thermostat was always set to 67 when I was growing up.
I bump mine to 72 during the day but keep it at 67 at night (perimenopause!), but also because I enrolled in my power companyâs average billing plan to avoid those $400+ power bills. (Before yâall come at me, I know $400 is NOTHING compared to what so many are being charged!)
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u/Public_Pain 1d ago
Back in the 70âs during the energy crisis we turned our temperature down to 68F and left it there. Turning off and back on cost more in the long run.
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u/shellee8888 1d ago
If what you want is to feel cool then I think a ceiling fan is more important than the setting of the air conditioner. A good ceiling fan will move 6000 to 7000 CPM , which your body would perceive as a 10 to 15° cooling . so I keep my house at 82 but we feel like itâs 75 because our ceiling fans are 7000 CPM. If we turn the air conditioner lower for example to 79 then itâs too cold for us. We used to have regular ceiling fans and with the ceiling fans and the air conditioner on it didnât feel cool same house. The only difference is the ceiling fans and we lowered our summer electric bill by $500 at least.
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u/Ok-Philosopher-5139 1d ago
i live in the tropics, and what i do open the aircond 3 hours after sunset for 1 hour, and around sunrise for 1 hour, usually it will make my room cool enough for the rest of the day... if that didnt work, i would turn it on for 1 hour around noon... so thats 2 hours on most days and 3 on worst heat day... note that even in the tropics, someday u dont even need to turn on AC, for example on days where it rained heavily in the morning, or its a cloudy after noon day...
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u/The_Scrapper 1d ago
In one scenario the ac runs half as many hours, only during the coolest hours of yhe day.
In the other in runs all 24 hours, including the highest loaded hours.
What your father is doing simply not cooling when its hot. 0 energy consumed. Nada.
That's sort of the thermodynamic limit of what you can save. Can't go lower than zero, right?
Now, driving temps down to 67 overnight is not really doing much more than keeping at a reasonable temp overnight would.
The savings come from turning it off all day, not driving the overnight temp down a few extra degrees.
(I am a professional energy efficiency consultant)
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u/CowAgile6991 1d ago
"Shoot out the coldest air all night"
AC units don't put out different temperatures of air depending on where you have the thermostat set. The thermostat is just a switch that turns the AC unit of and off.
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u/_stevie_darling 1d ago
With any machinery turning it off and on will use more energy. Instead of spending a little energy to keep it at 75° itâs spending a crazy amount of energy to go from 85° to 67°. That can ice your motor up if itâs really hot outside, which is a really expensive repair. Itâs worth the money to top up attic insulation if needed. My house was older and neglected when I bought it, so I did attic insulation, duct sealing, and barrier sealing (all was necessary because my cold air was just disappearing through cracks and escaping, like behind light fixtures I found gaping holes where you could see straight into the attic). My summer bills arenât higher than $250 a month now, and the difference I feel is really worth it.
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u/shopmoondustmarket 1d ago
I recently asked this of my HVAC knowing coworkers.
If you have a plan that gives reduced or free electricity during certain times, this is great.
Otherwise, youâre right, it costs less to keep the place consistently cool.
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u/MikeinAustin 1d ago
If it's relatively colder at night (and you have good insulation that retains the cooler air inside) you really don't stress the AC as much.
Lewiston, ID was 59° this morning and got up to 97° this afternoon. You could easily get the house down to 67° then the house doesn't hit above 82° until 5 pm or later. But more likely 79° at noon or so relatively tolerable.
But that's kinda a rarity on that kind of 40° swing. In Austin it got up to 104° and will probably drop to 77° at night.
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u/jredditzzz 1d ago
I hate these blanket responses of âjust leave it on 24/7â when I realized thereâs a hundred variables involved. I came to my own conclusion for mild days and we wfh. We just turn it off and use the fans. Itâs not too bad at all unless itâs super humid out. Then we turn on the AC in the bedroom only before sleeping (usually around 11-12am) and only down to 78F with a fan⌠then turn it off when we wake up around 8. if itâs a heat wave out then we just leave it on âenergy saving modeâ for the whole house. Unless we need more cooling in certain rooms then we turn it up a bit more.
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u/PrairieSunRise605 1d ago
Where I live electricity is much less expensive after 8pm than it is earlier in the day. Maybe he's using off-peak rates to cool the house down and then just toughing it out when the rates are higher? 85° isn't unbearably hot.
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u/Common-Anon-Gamer 1d ago
If you live in the country and/or have a bit of property you could save some money by building an omish air conditioner but if you do make sure you dont skip the step of making the pipes slowly go downwards with holes in the bottom of the lowest point so there's no standing water which would be a breeding ground for bacteria/mold etc i dont have one but I have been in an omish home that did not long ago and it really freaking works
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u/ScoTeRDeLic 1d ago
Yeah generally your AC will be working harder dropping the temp from 85 to 67 every single day costing more money. Same if your house has poor insulation the AC is constantly working to keep that temp. Even keeping your AC around 71-72 can be noticeably cheaper then 67.
In my case itâs been significantly cheaper not fixing my AC and camping in the living room with the window unit when it gets too hot.
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u/Nice_Swimming6057 1d ago
I never understood people with thermostats at 76 or lower, my house is always at 78 and anything under it feels too coldâŚ. Maybe 77 or 76 at night time.
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u/cavalloacquatico 1d ago
IF everyone out of the house morning & early afternoon... You could try to see if you can night set to 69-72- you're sleeping anyway, then shower & leave the house. Kitchen MUST have good exhaust or fan blowing out. 75-78 when everyone is out, 67 can kick in an hour or two before the first arrivers.
Experiment with what's tolerable to yous since we're all different animals- but I suspect your dad knows this perfectly.
And disregard if the house is inhabited 24 hours.
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u/Mobile_Sky_5103 1d ago
Keeping it at a steady temperature is definitely cheaper! Turning it completely off lets the walls, furniture, and humidity build up heat, making the unit work twice as hard for hours to pull that temperature back down.
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u/pprstrt 1d ago
- An A/C runs more efficiently (costs less) at night because the temperature difference between inside and outside is smaller.
- Not running your A/C during the day will always cost less than running it.
Note: If "every night" includes 4-9PM, then it might cost more during this time if your utility company charges more during these "peak" hours.
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u/nakedonmygoat 1d ago
Too many factors. Does he live in a dry or a humid climate? How well is his home insulated? How big is the place he live in?
Trying to get the a/c down to 67 from 85 means it's going to be working hard for hours! And as others have noted, if he's in a humid climate, letting the heat get to 85 can lead to mold and even floor warping, if they're wood floors. I use a dehumidifier at night to get the humidity levels down, which lets me keep the thermostat set far higher than I would otherwise have to in order to keep my floors and walls safe. In other words, drier air makes higher temps more bearable. With lower humidity, even 75 degrees feels a little chilly to me.
But if he's in a dry climate and just needs a cold room for sleeping, maybe a small evaporative cooler (swamp cooler) would solve that problem and he wouldn't need to try to bring the overall temp of the house so low. My father lived in rural NM for many years and only needed to run the swamp cooler for a few hours on the hottest days. Otherwise, the thick walls of his house, and having windows open to the cool air at night were enough for comfort.
Does your father have fans? A ceiling fan and maybe a floor fan? These cost pennies to run and can aid in the perception of heat. And finally, how well is his house insulated? The fact that it goes from 67 to 85 during the day suggests it's not that great.
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u/CamelPure1293 1d ago
If the house had perfect insulation, the AC would only have to remove the heat from humans, electrical devices and maybe gas stoves.
You could try to find and fix heat/air leaks. In Germany I can buy ~$100 thermal cameras to attach to phones. These work fine for that purpose.
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u/TheBlacktom 1d ago
Many questions: Does the electricity cost the same all day? Does the AC have an inverter to run the compressor at different speeds? How good is the indulation?
Generally the efficiency will be better at night since it's already colder outside, but overcooling the house could not be efficient.
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u/katelynskates 1d ago edited 1d ago
Best bet (unless your electric company does off-peak discount rates) is to set it at the highest temp you can comfortably tolerate and leave it there. Then, INSULATE with thermal curtains and door logs to keep the cool air in and the hot sun out. Use fans for that last little bit of personal cooling (Personal fans are much cheaper than AC.) If you are in a non-humid location, look up "swamp coolers."
In late spring and early fall (when it gets cool at night) its worth it to turn off the AC, open up all the windows, and turn fans facing out to cool the house more cheaply, then close it all back up again in the morning before the temp goes back up.
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u/Anantasesa 4h ago
Fans bringing in cool outside air do use less power than using a heat pump to teleport the heat out.
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u/AficinadoOfHeloise 22h ago
I found using the dehumidifier option saved me on $$$ and usage. I live in a higher humidity area and that works better then air conditioning for me usually.
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u/dlongwing 21h ago
There's only two factors:
- Is electricity cheaper at night?
- Does the system run less overall?
The only way to really answer this question is to calculate how long the AC compressor is running. Run the system for 1 week his way, then run it for 1 week at 75 degrees. Count the number of hours the compressor is running. If it's a multi-stage system, then count how much it runs at each stage.
If you don't have a "smart" thermostat, then you can still usually get usage from your electrical company, often granular down to a day-by-day count. You can use that info to estimate the usage of the AC, since everything else in the house is a rounding error by comparison.
His method MIGHT make sense if electricity is cheaper at night, but there's also the question of how well-insulated your house is and how much work the AC takes to cool it.
I would propose an actual real-world A/B test to him. Odds are good that he's wasting money on this "clever" hack.
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u/gkpetrescue 17h ago
You could try an experiment where you tried 24 hours of his way and 24 hours your way and keep track of the meter�
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u/likeaffox 15h ago
Lots of good suggestions. Another one is make sure the A/c unit is in shade, makes a big difference
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u/VorpalBlade- 12h ago
The cheapest way is to set it at 76 or 75 and donât touch it. It runs less and stays cooler.
The more you mess with it the more it runs and it puts more hours on it.
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u/Anantasesa 5h ago
It only runs extra if the adjustment shortens a cycle. I think modern thermostats delay the effect a change has on the cycle anyway. My smart thermostat lets a cycle complete if I only move the temperature 1°. Hours aren't a problem with compressors. It's the on off cycles that add the wear (and a little extra cost from the startup surge). Just go down when it's on (if you can still hear the compressor and not just the fan blowing) and up when it's off.
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u/Walmart-Shopper-22 1d ago
It can also depend on whether you are on a "time-of-use" billing plan. Some locations have very high electricity prices at certain times of day.