Monday, 8 April 2019

Seven years of solar power: How valuable is our solar energy ?

I've had small scale interest in solar power since the mid 1980s when I first put a few small panels on the top of my parent's home which were used to charge, amongst other things, the battery in my laptop computer. It wasn't until seven years ago, though, that we installed a large system on our home and the seventh anniversary of that system passed on April 5th.

Seven year summary
We have 16 panels each rated for 235 W output for a total of a 3760 W peak. In this part of the world we expect the equivalent of 925 hours of full sun each year which means that we would expect about 3478 kWh per year. In practice, over the last seven years our system produced 23601 kWh in total or an average of 3372 kWh per year.

Output per month over seven years. Red bars show our electricity consumption, blue bars show the production from our panels and the yellow line shows where we are in total now relative to where we began. Our production in is higher than our consumption on average, though obviously that is not the case in the winter. The kink in the yellow line from July 2018 is the period during which our inverter was not working (I repaired it myself)
Several things contribute to the slightly lower output relative to the estimate: Our panels are mounted at the angle of the roof and not at whatever the ideal angle might be, they face South West rather than directly towards the South. But perhaps the most damaging thing so far as the average is concerned is that our inverter failed last year and so we had no output at all for a couple of weeks during the sunnier than average month of July. The average for the first six years, not including the month when the inverter was not working, was 3410 kWh per year. Remarkably close to the estimate.

Return on investment
The monetary value of the electricity which we've generated is about 20 cents per kWh for that which we use ourselves and about 7 kWh for the excess that we export to the grid. We have exported about 1200 kWh in total for a value of €84 and consumed about 22400 kWh ourselves for which we would otherwise have had to pay about €4500. In total, then, about €4600 has been returned from our original €8000 investment. At the same rate of return the system will have paid for itself after 12 years.

Our new smartmeter. When the photo was taken
everything electrical in our home was powered
from the panels and 2 kW was being exported
to the grid.
If the inverter fails again and this time we have to replace it then that will of course increase the repayment time.

The panels should last much longer than the repayment period.

A smart meter
Something else which changed last year was that our electricity meter was replaced in December.

The new meter doesn't give us any advantage over the old mechanical meter which span backwards just as enthusiastically as forwards for nearly seven years. The electricity costs us the same amount either way around.

Of course it does offer an advantage to the electricity company because they no longer have to ask us to read the meter or visit to do so themselves.

The one interesting thing for me about the new meter is that it counts differently and presents more information. There are separate counts of kWh for both peak and off peak times (though we pay the same for both) and for inbound and outbound electricity. So now we can see how much electricity we consume at night time and we can see how much leaves our home rather than being consumed here.

Solar electricity is more valuable than average because it is
generated at peak times of consumption (source)
Perhaps after a year of recording this information I'll have something which which I can create an interesting graph. Thus far I have just three and a half months of data, all of it from winter and spring. There's not much contrast to see.

What has long been obvious of course, and is also visible on the data, is that our electricity is generated only in the daytime and then mostly on sunnier days. As it happens, solar electricity is valuable in the Netherlands because this daytime generation corresponds quite well with peak times of usage. So what we export is almost certainly of use to someone.

The first thing I made the prototype
hardware do was wiggle a GPIO and
then send serial data as shown here.
Software for a smart meter
I'm late to have a smart meter installed but as it happens, I spent a few months a couple of years ago working on software for the prototype hardware of one of several competing manufacturers. I'm almost certain that the meter that I have now is not related to the manufacturer for whom I did the work, so unfortunately my code is not involved in reporting my own electricity usage.

It was quite an interesting project for a while because it was like a return to the old days for me, when I worked on 8 bit processors and embedded software. This work was with what for me was a new processor, the RL78. An 8/16 bit design. The RL78 assembler is styled so that the source code looks a bit like that for the Z80, but the processor is really completely different. In total I had 32 kB of flash and 4 kB of RAM to work with. These days that doesn't sound like much at all but that means this device has about the same amount of memory as the guidance computers which took Apollo to the moon. But they have this in a finger nail sized package which consumes micro-watts, and the little RL78 is many times faster.

Power for a home and a business
We generate more than enough electricity to power both our home and our business, though of course I always point out that we're not actually doing so at night, or on darker days in the winter. Anyway, if you want a support a solar powered bicycle business which uses no powered vehicles then you can do so here:

No comments: