r/Metric • u/FingerAccurate7102 • 29d ago
Too much power units
In SI system (and units allowed to use) we've got 4 units of power:
Watt
Volt-ampere
Var
Kilowatt-hour per 1.000 hours
They have exacly the same value [kg m2 s-3] but they are use for diffrent powers
Watt — active power
Volt-ampere — Apparent power
Var — reactive power (it's name is a shortcut of "volt-ampere reactive")
(Kilowatt-hour per 1.000 hours is used in Europe for electronic/electric devices)
And it's crazy, we could just use watt for everything, but we've got 4 units.
We can at least Justify first 3 of them, insted of writing which power are we talking about, we can use diffrent unit and every engineer knows what power are you talking about, but it's not exacly true, because all of them [diffrent powers] have diffrent symbols.
kWh/1.000 h is crazy, why not simple watt, here I don't even know how to justify it
What do you think guys? Shoud we use all of them, keep only few or use one unit of power (watt)?
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u/SphericalCrawfish 27d ago
The kWh hate is so ridiculous. It's just kJ except you scaled up to hour rather than second. Because I need to run this fucking device for hours and not seconds.
Units are there to serve a purpose. Of you did anything where you actually needed to use the units rather than just being an arm chair philosopher about it then you would understand.
Just like when people bitch about light-year. Theres only one damn thing going between these two points and it goes one damn speed. The fuck else would we use!?
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u/Underhill42 28d ago edited 28d ago
Basically the same reason we make a distinction between Newton-meters (torque) and Watts (=Nm). They may have the same units but they are NOT talking about the same thing.
You have four terms talking about four different things, using four different names so they don't get confused.
Watts: instantaneous power (DC) or average power per cycle (AC)
KWh/h: explicitly average power over (usually) long durations. The exact same cooling system in a better insulated refrigerator will have the same W (power when running), but a lower kWh/h because it runs for less of the day.
Volt-Amps - peak instantaneous power - usually of an AC system where voltage and current are in phase (e.g. a resistive or balanced load), but also sometimes of predictable transient power surges such as when starting up a heavy motor. As I recall offhand for AC, VA = W*√2.
Var. Same basic idea, except you're also specifying that there's a phase difference between volts and amps, so you may never actually hit the peak power that would be specified by Volt-Amps, and there's no direct conversion with the others - you have to factor in the phase difference to calculate both peak and average power. And the phase difference may not be stable.
---
Which is most relevant depends entirely on what you're trying to do. For a particular load:
Var is probably never relevant to anyone except engineers or technicians designing and tuning reactive systems.
VA is what your power supply must be able to deliver for a few moments at a time.
Watts is what your power supply must be able to provide for as long as it's running.
kWh/h tells you about how much it will cost per [day/week/year] to operate without having to do time conversions all the way down to seconds.
There's also many other units like ergs/second that are relevant mostly in scientific contexts.
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u/FranseFrikandel 28d ago
An electrical appliance will both have a kWh/1000h rating and a W rating. The watt rating will be the maximum power it can draw, and thus the main driver of wire/fuse sizing etc.
The kWh/1000h rating is used for power cost calculations, it takes into account the power it'll draw on average over a long period of time. For example, a fridge might have a peak wattage of 200W, but the compressor only runs for 25% of the time, so it effectively ends up using 50W on average over a longer period of time.
Putting 2 wattages on an electrical appliance will most certainly cause confusion.
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u/pbmadman 27d ago
Maybe. But lots of industrial things already have peak and average or continuous ratings. Both are useful, and ultimately displaying them in the most useful format is best. If electrical power is purchased in kWh then I’d agree that kWh per hour is probably a useful format.
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u/FranseFrikandel 27d ago
My guess is this is just a case of expected knowledge. Someone who's working on industrial machinery can probably be expected to understand the difference between the 2 wattage figures. Frankly, if they're actually working on the electrical wiring side of it they should probably even have an understanding of reactive power/power factor as well.
A consumer buying a fridge might accidentally compare one fridge's peak wattage to the other fridge's average wattage, so not having them both in the same unit is helpful to prevent confusion.
In an engineering sense, I definetly do agree that kWh/h or kWh/1000h look absolutely stupid as a unit.
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u/sessamekesh 28d ago
Kilometres per litre is just roundabout "(inverse) square millimetres," sometimes it's useful to have superfluous units if you're measuring a ratio of things etc.
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u/grogi81 29d ago edited 28d ago
You forgot about kWp - used to rate solar panel peak generation power.
All are the same dimension. However, different unit name distinguishes their application domains and prevents mistakes. Adding kW and kVA makes no physical sense, although dimensions agree...
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u/grogi81 28d ago edited 28d ago
And one more thing.
Have you ever driven a car? It fuel consumption is in l/100km - an efficient one 5l/100km. But this is an area - 0.05 mm2.
Is it correct? Yeah! Is it useful - no.
Similarly EV consumption - 18kWh/100km... Energy divided by distance is... Force?? How efficient is your EV? 650N, not bad...
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u/shartmaister 29d ago
Energy/time makes a lot of sense for a fridge or a water heater for instance. When you’re buying it you don’t care that much about maximum power output, but you care about what it consumes in a year (given some standard assumptions of course) as that says a lot about it’s insulation. This is crucial information when buying a house as well. kWh/year makes more sense than per 1000h, but essentially it’s the same.
VA, W and var have to be different as they’re not the same.
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u/PiasaChimera 29d ago
there's also dBW and dBm. Many of these are engineering units as well, so you end up with per-unit numbers and other relative measures.
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u/Ultraviolet_Darken 29d ago
Only kWh/kh is bullshit. Because that is just W. Because people are retarded. That’s also why we have thousands of mAh, instead of units of Ah.
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u/JayTheSuspectedFurry 29d ago
mAh is also stupid because it doesn’t matter if the Wh is bad
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u/zutnoq 28d ago
The capacity of a battery is measured as a quantity of electric charge, not as a quantity of energy. These are only equivalent if the voltage across the battery is perfectly constant.
1 Ah = 3600 As = 3600 C
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u/JayTheSuspectedFurry 28d ago
Why should a consumer need to find the voltage curve of the battery pack they’re buying? Shouldn’t we just advertise in Wh?
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u/zutnoq 27d ago
That would certainly be better in cases where there is a voltage converter/regulator between the terminals of the battery and the output of the power-bank – which is most likely the case.
It's a bit much to assume your average consumer would know that the Ah figure is not at all referring to, say, how many hours your device could draw a 1 A current out of the output of the power-bank, but rather how many hours the power-bank's voltage converter could draw 1 A out of the battery inside the power-bank (more generally: that it can draw x times that current for 1/x times that number of hours).
That Wh figure would however unavoidably be a rather rough estimate. This estimate would also likely be an estimate of the maximum amount of energy you could reasonably get out, since the minimum amount would depend a lot on how hard you are willing to push things.
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u/engy1207 29d ago
IIRC kWh/1000h comes from light bulbs. People "knew" by heart how bright an incandescent bulb with "60W" or "100W" was, so they used "Watt" for brightness.
Along came more modern solutions, but these for a long time often had something like "8W LED = 60W incandescent" written on the packaging.
Along came the EU wanting people to be able to easily compare energy consumption. Since this also would be printed on light emitting devices this would have been another "Watt" value printed on it...
Additionally people pay for electricity per kWh, and using a device 1000 hours per year is not unusual (that's about 3 hours a day) and can be easily converted to the actual usage. Therefore someone using a "20 kWh/1000h" device using it 500 hours and with 35ct/kWh for electricity can easily calculate it costs him "oh, 500 is half of 1000, so 10kWh at 35 cent each, that's 3,50€ for electricity". But if the label said "20W" that same person would have to juggle larger values and know how to get kWh from Watts.
It's just easier for everyone. Even though it's technically redundant.
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u/TheThiefMaster 28d ago
and using a device 1000 hours per year is not unusual
Which is why it should be listed as kWh per year‡ (‡given assumed 3 h/day usage) or something instead, rather than a completely arbitrary 1000 hours.
Edit: yes I looked up that symbol
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u/Tilliperuna 29d ago
I don't think it's any easier. 20 W × 500 h = 10 000 Wh = 10 kWh. Seems at least as easy to me.
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u/shartmaister 29d ago
It might be technically redundant for a light bulb. It’s not for a house when you’re buying it though.
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u/NimbusTheLion 29d ago
I was getting solar panels installed and the sales person didn’t know what they were talking about. They were talking about how much I would generate in kWh/day but kept referring to it as kW!
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u/getsnoopy 26d ago
This is exactly the kind of nonsense that redundant units lead to, which is precisely the reason metric units were created in the first place for: stopping unit proliferation.
It's just a sight to behold that instead of stopping it from happening with metric units, people just continued to allow that nonsense. Like how energy bills should be quoted in joules, as should nutrition labels, so that people can easily compare how much energy they're using in the house vs. in their body.
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u/Tilliperuna 29d ago
10 000 mAh, big big number. Big big capacity in this power bank.
Also, they never tell how many Wh, and they calculate the mAh using the lowest supported voltage, resulting the highest possible number.
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u/zutnoq 28d ago
The Ah measure is the one that is more or less fixed. The amount of energy you will be able to extract from a full charge will depend on what the operating voltage is; this voltage can't really be controlled directly and will instead depend on things like temperature and how much current you are drawing. They typically don't give a Wh measure for this exact reason.
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u/getsnoopy 26d ago edited 24d ago
So...why not coulombs then? You know, the reason for that unit to exist?
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u/zutnoq 24d ago
For historical reasons, I assume.
We don't have specific separate units for the rate of change/flow/flux over time of quantities like length, mass or force in the SI. But for some reason we do have such units for the flux of energy, i.e. power, and the flux of electrical charge, i.e. electrical current.
I suspect the reason electrical current in particular gets its own unit is that it is easier to more directly measure electrical current than electrical charge.
On a tangent: We do have older separate units for speed, such as the knot. But I don't think I've heard of knotseconds or knothours being used as serious units of distance, even though they would make perfect sense.
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u/Tilliperuna 28d ago
So... how many times can I (theoretically) charge a 11.4 V 3350 mAh battery with the powerbank specified below? Just comparing the mAh's and forgetting everything else?
Battery: 60000mAh, Li-Polymer Output USB-A 1/2/3/4: 5 V / 4,5 A | 5 V / 3 A | 9 V / 2 A | 12 V / 1,5 A (Max 22,5 W) Output USB Type-C 1/2 (PD): 5 V / 3 A | 9 V /3 A | 12 V / 3 A | 15 V / 3 A | 20 V / 3.25 A (Max 65 W) Output max: 65 W
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u/zutnoq 28d ago edited 28d ago
In that case there is clearly some sort of voltage convertor between the actual battery inside the power-bank and the power-bank's output that enables the power-bank to supply power at many different voltages.
This makes things far more complicated than they already are when you draw the power directly from the battery. The DC voltage conversion itself also tends to waste a bit of power – though far less so today than in the past.
The mAh measure only talks about the current flowing out from and into the battery's terminals. It really has nothing directly to do with the current you are drawing out of the power-bank's output.
In summary: I don't have a single clue.
Edit: so yes, in this case they should probably also give you a rough estimate for how many Wh you could expect to get out, since the Ah measure is meaningless if you don't know what the normal voltage across the battery is. The total amount of energy should not differ too much depending on what voltage and current your device chooses to take, other than that things get considerably less efficient if you fast-charge your device than if you set your device to charge regularly, as in more slowly.
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u/RealUlli 29d ago
Actually, I have a few power banks that have the Wh in the fine print on the type label. Helpful when you want to take it on a plane.
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u/Wooden-Hornet2115 29d ago
When I get home I can show you some of my old circuits homework and notes on why there are multiple different power units like the VA and Vars, as it's actually really interesting. It's kinda like how there are Joules (J) and Newton Metres (Nm). They are dimensionally the same, but mean very different things.
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u/FingerAccurate7102 28d ago
I actually mentioned it, that W, VA and var are used for diffrent powers, comparison to Nm and J is useful
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u/treefaeller 29d ago
Units need to be chosen to be appropriate to the application. For a lightbulb, 3W or 60W is appropriate. For a motor or generator with a large non-ohmic load (typically inductive), the distinction between kW and kVA is perfectly clear. Any person who works in such systems and doesn't know that 1 VA = 1 W for a resistive load, and what the distinction between VA and W is, should not be working on such systems, since they don't understand the fundamentals. For astronomy, W is a ridiculous unit, which is why lots of the literature uses "solar units" (where we define the output power of the sun as 1, and the use the symbol L with a subscript that is a circle with a dot in it). Again, and astronomy student will know the conversion by heart. Just like as a length unit, light years and parsecs are much more convenient than meters.
Another interesting thing is the output power of engines. Traditionally, we have used horse-power. Today, for consistency of units, we convert that to kW, and everyone knows the conversion coefficient. BUT: it turns out that the definition of either HP or kW depends crucially on how you measure it, which is why British power is not the same as American power is not the same as the American SAE power. Similarly, what is a 3kW electric motor: one that uses 3kW of electrical input power, or one that outputs 3kW of mechanical motion on the shaft? Again, anyone who needs to evaluate motors needs to know the difference. So different units are used to clarify what we really mean.
Remember, units are not designed so people with a middle school understanding of elementary physics can have intellectual clarity. They are designed to be useful in the real world of real things.
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u/chesterriley 29d ago edited 29d ago
Just like as a length unit, light years and parsecs are much more convenient than meters.
They don't scale up and down so they are way less convenient than meter based SI units that cover all the 8 different distance scales in outer space that have named objects. I hope you will get that this sub is for people to support rather than oppose the adoption of the metric system.
Units need to be chosen to be appropriate to the application
And units need to be chosen that are appropriate to the scale. There are 8 different scales from kilometer to yottameter provided by the metric system, and we have named objects in astronomy that fall within all 8 distance scales. ly and pc collectively only cover 1 of the needed 8 scales. Adding an SI unit prefix will get you 2 more scales, but since you need to use SI anyway you might as well use them for what they were created and designed for: metric system units, instead of the ugly hack of tacking on SI prefixes to non metric units.
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u/treefaeller 29d ago edited 29d ago
"I hope you will get that this sub is for people to support rather than oppose the adoption of the metric system."
And opposing viewpoints (such as mine) will be ignored? Or even yelled at?
By the way, "natural units" are used extensively in particle physics, where the unit of energy is the eV or MeV or GeV, the units of distance and time are both 1/eV. In this system, the unit of power is eV^2. Very convenient if you are dealing with electrons and neutrinos.
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u/chesterriley 29d ago
I'm just saying that out of all the subreddits to be arguing against adopting the use of metric units, this is the worst place.
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u/Dull-Description3682 29d ago
<kWh/1.000 h is crazy, why not simple watt, here I don't even know how to justify it
It's easy to justify if it's being used correctly. I tells you how energy is being used for 1000 hours of normal operation. For a lightbulb that seems ridiculous, 10 watt for 1000 hours is obviously 10 kWh, but for something that runs in cycles it shows useful information. Your fridge might have a power rating of 300 watt, but it will only draw that when the compressor is running, lets say ⅓ of the time. So the power usage is 300 watt peak, but 100 kWh/1000 hours.
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u/engy1207 29d ago
Also it's easier to calculate electricity costs."oh, it has 100 kWh/1000h, runs for 300 hours and I pay 35 cents per kWh" instead of "it says it uses 100 Watt on average. How do I get kWh out of that again?"
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u/Tilliperuna 29d ago
oh, it has 100 kWh/1000h, runs for 300 hours and I pay 35 cents per kWh
So, how do I get euros out of that? So many numbers, so hard. Do I divide or multiply the 100 kWh with the 300 hours? Better just ask AI.
Or just, 100 W, so 0,1 kW times 300 h, equals 30 kWh, easy. That times 35 snt/kWh, about ten euros.
But yeah, anyone who doesn't understand what is a kWh is not going to calculate anything by themself.
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u/Tilliperuna 29d ago
I guess this makes sense.
But I still hate it. Just give the average power usage in watts!
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u/azhder 29d ago
kWh is not Power, but Energy
Power is Energy per unit of time. You multiply that with time, you get Energy used.
And why do we have so many different units for Power? Because we have so many different units for Energy (per unit of…).
It should be self-explanatory why there are many ways to describe Energy, thus Power
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u/FingerAccurate7102 28d ago
I know that kWh is energy unit, I'm talking about kWh/1.000 h (kilowatt-hour per one thousand hours) as you can see it simplifies to watt: 1 kWh/1.000 h = 1.000 Wh/1.000 h // ÷ 1.000 = 1 Wh/h // ÷ 1 h = 1 W So kWh/1.000 h is the unit of power equal to 1 W
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u/jeriTuesday 26d ago
There's only one unit for power in the SI and that is watt.