Saturday, 5 April 2014

a new view of solar stats

March is over, so we have another month of solar power generation statistics.  Looking at the sunniest day (as determined by total power generation) of each month, we see:

The horizontal time axis runs from 3:00am to 9:00pm GMT. The vertical axis runs from zero to 8kW. The orange regions indicate the minimum, lower quartile, median, upper quartile, and maximum generation at that time, over the respective month.

The different amount of sunlight month on month is clearly visible.  The total power generated on the sunniest day each month (essentially sunny all day) was 26.5 kWh on 13th January, 41.4 kwH on 16th February, and 52.0 kWh on 24th March.  In the March plot we start to see saturation: we have an 8kW system, and so the top of the curve flattens around noon as the system generates at full capacity.

Of course, one of the reasons we want a solar power system is to use the electricity we generate.  These plots show only generation, not usage, because that's the data the solar power system provides.

So we got another meter: a Wattson monitor, that shows not only power generated, but power used, too.  We download the data weekly as it requires an actual cable to connect the Wattson to a computer (the generation system by contrast shares its data via Bluetooth).

Here the horizontal time axis runs from midnight to midnight GMT/BST. The vertical axis runs from -8kW to 8kW. The region above the axis represents our usage: green is generated usage, red is imported from the grid. The region below the line is surplus generation exported to the grid.  (So the green area corresponds to total generation, or the black line in the plots above.)  During the day, usage tends to track generation.  That's because we have a system that pours excess generation into the immersion heater, up to a maximum of 3kW.

The early evening spike at the weekends is dinner being cooked in the electric oven; during the week we usually use the gas hob.  Looking closely you can often see a small red spike around 7am (when generation is just starting); this is the kettle for my morning coffee.  On the 17th, 18th, 19th, and 26th there is a lot of red: we had the “boost” on the immersion heater switched on, because the gas boiler had a fault, so we had no gas heating.

So just eyeballing these charts, it looks like we might be saving about a third of our electricity bill, in addition to what we get paid for the generated power.  This proportion will probably increase in the summer as the number of hours of daylight increases.

Then, of course, there is all the entertainment value of writing Python scripts to generate the charts, examining said charts, and speculating on the shape of future charts.  Great value for money all round!

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