Friday, 26 October 2012

An extra 2,000 yen

Talking electricity again: this time the bill we have to pay. After the delight of earning ten thousand yen more than expected in July-August, it was a surprise to see the amount we pay go up by a couple of thousand yen in August-September.

There is only one wire going in and out of our house, and two meters outside measuring what goes in and what goes out, so if we use electricity during the day time while the panels are generating, we will use our own power and sell less. 

The electricity bill gives a breakdown by time, and we usually buy the most electricity at night time, about 50% more than we buy in "at home time"  which is in the morning, 7am to 9am, each evening, 5pm to 11pm, and all day Saturday and Sunday 7am to 11pm. We buy very little during the day as we are usually generating far more than we need. Night time is 11pm to 7am, which is 8 hours per day everyday. Considering weekends, at-home time averages a little over 10 hours per day, and day time a little under six. Although we use much more electricity at night, we pay much less for it, and at-home time is the most significant part of the bill we pay.

From the solar panel monitor we can also see how much we consume and how much we generate. At the moment this has to be copied manually, and detailed information is lost after a month, but I hope one day to be able to download the data onto a computer. That's another story.

So why did we use so much more? I don't think we did a lot of cooking or washing, which are two of the main power users. We'd need to charge a few hundred phones to make that difference, and leave the TV on all the time. If we had been using a lot more hot water, the night time usage would have gone up but not the at-home usage, as the boiler heats water at night time. 

Another thing that appears from the bill is that this was a 33 day period, containing two weekends, so there were several more hours of at-home time than usual. Perhaps this is part of the answer. Also, while it was a couple of thousand yen higher than the August bill, that was almost a thousand yen cheaper than the bills for the past three months,

I wondered whether it was the extra load on the ventilation system because we hadn't changed the filters. A closer look at the numbers shows that the main difference was in the at-home time, where we were using around 140 watts more than the last few months. In fact at night time we were using less, if anything, so it doesn't look like a constant electricity user. The ventilation system is on all the time, so we would expect to see both night time and at-home time consumption increase. Day time consumption, as far as the electricity bill is concerned, would be lost in the fluctuations of generation. 

The only thing I can think of that we had changed was switching off the monitor from the solar panels. Before, it had been set to switch on whenever we were generating, and part of the display was encouraging us to save more electricity, or praising us if we were meeting a target. I've switched it back on again, and perhaps that will encourage us to use less electricity.

This echoes something David MacKay said in his fantastic book of energy exposition and explanation,   Without Hot Air, available in full online: "Since I started paying attention to my meter readings, my total electricity consumption has halved" (p. 156)

I had only switched off the monitor to save electricity!