First results are in for our solar panels!

[StockBeard]

We moved in to a new house at the end of July. The house is properly insulated, has solar panels and I wanted to share my numbers.
TL, DR: net result is income positive for us in the month of August.

Background:
As a short summary, this is a house built with fairly high insulation standards (for Japan). The makers are Yamato Juken ( https://www.yamatojk.co.jp/ ) I am not affiliated with that company, and keep in mind that I have lived in that house only for a month so far, so don't take any of this post as a particular recommendation for your own house choices. (We had also considered a company named ichijo who have roughly the same insulation standards)

In August, we had the air conditioning running at 25.5 C the whole time. That is, 24h a day, 7 days a week. This is not something I have ever done in my life before, but The house is built with a ventilation system that cools the entire house with a single A/C unit ( https://www.yamatojk.co.jp/yucaco/). On the surface, this is touted as slightly lower energy costs (because only one A/C unit) and that might be true. But cooling down an entire house with just one unit has a huge latency: on our first day, it took roughly 10 hours for our bedroom to go down from 36C to 26C, so we barely slept. After that the house was at the right temperature, but we reached the understanding that if we turn the thing off, then next time we turn it on it will take half a day again to reach an acceptable temp

The house has a 5kw set of solar panels (specifically, 5.3kW). Tepco pays us 24 yen per kwH as far as I remember (this is a national set price, probably easy to find online. I think it's 24yen)

The solar panels are configured in a way that we use solar energy first, grid second. Anything we generate in surplus is sold to Tepco. So, on a typical sunny day of summer, we generate more than we use from 6am to 3pm, and generate close to zero outside of this (which is when grid energy is being used). The result is that I'm not totally sure how much energy we actually use and how much we generate, although I do have a counter in the house that seems to give me an approximation.

However there are numbers I can share for sure, which are my electricity bill and the money we've received for the solar energy.
Electricity bill for August: 8'700 yen
Solar energy sold to Tepco: 9'000 yen

So, in August, not only did we pay zero for our energy use, we actually got paid. (300 yen!)

Now, as to actual energy consumption, according to my counter, my estimate is that we used about 16'000 yen worth of energy through the month (which even with good insulation is not entirely surprising given that the A/C was on the whole time). 7'000 yen worth of energy was directly used from the solar panels, plus the 8'700 yen we paid to Tepco. The solar panel themselves generated roughly 16'000 yen worth of energy. 7'000 were used by us as I just described, 9'000 we didn't use and sold to Tepco. Just a rough estimate as I'm not sure how much I can trust the in-house counter.
16'000 yen per month is what friends told me they use in energy for their family house in Japan, but that's using the air conditioning only in the middle of the day.

We have turned the air conditioner off today, and it might stay that way if September continues to be cool, so I'm expecting our energy consumption to go down for the next few months compared to August. But I would also expect energy production to go down if the weather remains cloudy. So our final bill for these months might end up being more expensive than August.

Nonetheless, so far so good, as I was only expecting the solar panel to reduce my electricity bill to something comparable to a manshon, when in fact it seems this is taking it down to Zero. (not counting the actual solar panel costs, though. Don't get me wrong, technically the discount we get on electricity is just paying for those, and it will take a decade to break even?)

[captainspoke]

That's a nice run down, and interesting to read. I'd like to have a new house like yours someday (wife undoubtedly would, too!), but don't know if it's in the cards.

I've read recommendations for Ichijo by someone on reddit who seems to know their stuff, and something on par with that would be wonderful. Congratulations!

[RetireJapan]

We almost went with Ichijo, and found them okay if somewhat inflexible. Ended up getting a small local place to do a total renewal instead (with me prodding them to do better insulation and find better heat-exchange ventilation). Very happy with the result.

[vapid]

I will make a separate thread related to this, but I wanted to ask how the performance has been the last few months?

I am quite interested in the economics for wintertime; specifically is it cheaper to run your AC/heat unit 24/7 vs turning it on and off as needed (or when you are present in the house). My general understanding is that with a well-insulated house (as yours appears to be), keeping the heating unit on is the more cost-efficient option.

[vapid]

Has anyone, who is a homeowner in Japan, consider the following company:

https://trende-eng.com/en/hot-denki/

[Bushiman]

I am quite interested in the economics for wintertime; specifically is it cheaper to run your AC/heat unit 24/7 vs turning it on and off as needed (or when you are present in the house).

Hey vapid... Have you found out anything regarding this?

[Beaglehound]

Regarding air con on all the time or not, we have just had a house built and over a beer at the weekend the builder was telling me that with the most modern systems it is indeed more economical to keep them on all the time. What that actually means in practice I am not sure, e.g. does it just apply to main living space or bedrooms too, and what temperature should it be set to when you go out. Am having a meeting next week to find out how to use everything in the house so will ask and report back.

[Bushiman]

Regarding air con on all the time or not, we have just had a house built and over a beer at the weekend the builder was telling me that with the most modern systems it is indeed more economical to keep them on all the time. What that actually means in practice I am not sure, e.g. does it just apply to main living space or bedrooms too, and what temperature should it be set to when you go out. Am having a meeting next week to find out how to use everything in the house so will ask and report back.

Oh, great! Keep us posted...
I live out in Nagano and even though the house is relatively new (8yrs), having to re-heat the lounge & kitchen each morning I'm guessing takes a lot of power... If the AC was left on 24/7, not just the air but all the furniture, floor etc. would retain some heat/warmth themselves over time... It would be so nice... I'm tempted to just try it, but afraid of the monster bill at the end of the month... I'd like to keep it around 23~24ºC daytime, and I'm guessing 15~18ºC at night and when we're out...

[Beaglehound]

We are up in Hida and have hitherto relied on toyu stoves so I know where you are coming from regarding the heating and the desirability of living in a constantly comfortable space. I was ready to fork out for a central heating system as understood in the UK but the builder assured me air con would be enough and useful in summer too.

[StockBeard]

I will make a separate thread related to this, but I wanted to ask how the performance has been the last few months?

Money received from the solar panels:
Aug: 9'000 Yen
Sep: 7'944 Yen
Oct: 9'192 Yen

Electricity Bill:
Aug: 8'728 Yen
Sep: 10'836 Yen
Oct: 9'480 Yen

(I believe we used the A/C for cooling until early November? Definitely were still using it in October)

I am quite interested in the economics for wintertime; specifically is it cheaper to run your AC/heat unit 24/7 vs turning it on and off as needed (or when you are present in the house). My general understanding is that with a well-insulated house (as yours appears to be), keeping the heating unit on is the more cost-efficient option.

Regarding air con on all the time or not, we have just had a house built and over a beer at the weekend the builder was telling me that with the most modern systems it is indeed more economical to keep them on all the time. What that actually means in practice I am not sure, e.g. does it just apply to main living space or bedrooms too, and what temperature should it be set to when you go out. Am having a meeting next week to find out how to use everything in the house so will ask and report back.

As far as energy usage is concerned, and whether it is recommended to keep the heater on constantly or not:

I am no expert so take everything below with a grain of salt, but as far as I understand, "keeping it on 24/7 is more cost efficient", whether heater or cooler, is a (marketing?) myth, that has been told for decades and has been packaged every time with the same "with modern systems..." pitch.

The best analogy I've seen is that your house is like a leaking bucket (somewhat explained here: https://heatinghelp.com/blog/the-leaky-bucket/ ), that you are trying to fill with water (the water represents the heat. The hole represents the heat loss in your house). It is much more cost effective to fill the bucket only when you need it, than to "feed" water to your leaky bucket through the whole day when you're not around, just for the purpose of having it full when you need it. At the end of the day, what you pay is the "loss" through the hole, which can only comparatively increase if you feed it when you wouldn't need to.

Of course we're only talking energy efficiency and cost here, not the resulting impact on your comfort level. But I don't see how it can ever be a good idea (financially) to keep those running while you are out. A timer/thermostat is the solution you are looking for if your goal is to have the house at the right temperature when you get back home (and if that's what the house builders are talking about when they say you can leave it on constantly, that's a bit of a stretch to call this the A/C)

So, at best, in the specific scenario of a very well insulated house, you might approach the same efficiency by keeping the A/C on constantly (as if you were turning it on and off as needed), but there is not a scenario in which it becomes more efficient than turning it on and off as appropriate. Less of a hassle, maybe, but how much you are ok to pay for that comfort is for you to decide.