We got the first electricity bill, which was for 45,000 yen. When I say bill, actually that's what they are going to pay us. The actual bill--what we have to pay them--came a few days later for 12,000. It should be a bit less than this once we get the heating and hot water sorted out.
The 45,000 yen is based on 951 kilowatt hours, for each of which they pay us 48 yen. The counter on the display panel went over 1000 kWh on 24th January, so in the first month we generated slightly under a megawatt hour and we sold almost all of this.
When we buy electricity, there are three different time zones:
Off peak "Night time" is between 11 pm and 7 am at 9.33 yen per kWh.
"At home time", 7 to 9 am and 4 to 11 pm, is 21.23 yen per kWh.
"Day time", 9 am to 4 pm is 31.43 yen per kWh.
In terms of our electricity production, although there's a fair bit of difference depending on cloud cover and temperature, it follows the geometry of the sun's arc across the heavens, and in the middle of winter comes in from around 7 in the morning till 5 in the evening with most of the generation between 10 am and 2 pm.
There is a single wire coming into the house from the grid with two meters on it. When we're using more power than we're making, electricity comes into the house, and adds up on the spangly digital meter. When the solar panels are making more power than we're using, electricity goes out of the house and adds up on the clockwork meter. I thought the reason for their meter being different to our meter was some kind of discrimination, or that it would be easier for them to misread and pay us less, but apparently it's because the meter for electricity we buy has to distinguish between electricity at different times, while the electricity we sell is all at the same price, so it just needs to clock up.
Ideally, as far as our domestic finances are concerned, we want to only use electricity during offpeak hours - 11pm to 7 am, when we can buy cheap electricity, and each kWh is costing us around 9 yen. Then, while the panels are generating, we don't want to use any electricity as we would lose 48 yen for each kWh that we don't sell.
Of course this is not entirely possible, and sometimes we need to use electricity in the middle of the day. We're also talking about a few yen here and there, and it may not make a big difference, although if those yen are adding up every day over the next ten years they will.
None of this is going to make a big difference to global warming or carbon emissions. Whether we use the electricity at night time or day time is not going to change the total electricity consumption. In fact in view of line losses, it may be better for the environment for us to use all of our own electricity and stop using cheap night-time electricity.
The only way this penny pinching is likely to help in the grand scheme of things is by increasing awareness of our power consumption.
