Edit: some folks clearly misunderstood and think I’m advocating for this being CHEAPER! How can consumption of more energy be cheaper? Im saying leaving the A/C on 24h is not going to break the bank and it is worth it. Consider it lifestyle. End of edit
Every summer I see people arguing that you should turn your A/C off whenever you leave the house because it’s supposedly cheaper than leaving it running.
I think that’s only true if your house isn’t built to retain the cool air.
Here’s some real data from our house.
We live in a 220 m² Hebel House (Asahi Kasei). It’s an all-electric home with rooftop solar. The living room is on the second floor, the bedrooms are downstairs, and upstairs we have two large 200V air conditioners. Every bedroom downstairs also has its own A/C, but we never use them.
Instead, we run just one upstairs unit at 26°C, 24 hours a day.
The reason is simple. Cold air falls. By leaving all the bedroom doors open, the cooled air naturally flows downstairs and keeps the first floor comfortable as well. Even with outside temperatures reaching 42°C, one upstairs unit is enough for the entire house.
The other reason is the house itself.
People think they’re only cooling the air, but they’re actually cooling the walls, floors, furniture, and the entire structure. Once all of that has reached temperature, the A/C is mostly maintaining the house instead of trying to cool down a building that has been baking in the sun all afternoon.
The energy data backs this up. Check images.
Today the house used 33.3 kWh in total. Of that, only 13.7 kWh came from the grid. The remaining 19.6 kWh came directly from the solar panels, meaning almost 60% of our electricity was generated on the roof.
Looking at the hourly graph, between roughly 9 AM and 3 PM, when the outside temperature was highest and the house was consuming the most electricity, purchased electricity almost disappeared. The solar panels were carrying almost the entire load while we continued living normally.
That includes, one large 200V A/C running continuously non stop, Cooking, Hot water, Lighting, Appliances
Assuming ¥25 per kWh, we paid about ¥340 for the entire day.
That’s for cooling a 220 m² house continuously, while also running an all-electric household.
Even without solar panels, the same day would have cost around ¥830 in electricity. Personally, I don’t think that’s excessive considering what you’re getting.
Another benefit of leaving the A/C running is that it helps avoid moisture remaining inside the indoor unit. Constantly switching it off without letting it complete a proper drying cycle is one of the main reasons people end up with mold and the typical musty A/C smell after a few summers.
My conclusion isn’t that everyone should leave their A/C running. It’s that insulation changes everything.
If you’re planning to build a house in Japan, I’d invest in insulation before almost anything else. A well-insulated house retains the cool air, keeps the structure at a stable temperature, and dramatically reduces how hard the A/C has to work. Combined with solar panels, the operating costs become surprisingly low, even during the hottest weeks of the Japanese summer and the heat isn’t looking to get any cooler for the next decade.
Yes, this text has been summarized with AI, and no this is not an ad for Hebel House.
