Tuesday 4 September 2012

Photo-voltaic cells: free money

We have a 4KW photo-voltaic installation on the roof, installed most efficiently by Llani Solar just over a year ago.

The annual statistics augur well for the future: the picture shows a plot of generation (slightly smoothed with a 5-day moving average) (blue) and a best fit sine wave (red). It also shows a similarly smoothed plot of use of the external supply (yellow) overlaid with another "best" sine wave (green).

The two sine waves are (click to enlarge)

(It's reassuring that the offset coincides almost precisely with the spring equinox - do note the lamentable weather in June). This implies an average daily generation of 9.79KWh; the FIT is 41.3p/KWh, so we grossed 366 x 9.79 x 0.413 = £1480 tax-free. Ha!

The external usage is slightly foxy to analyse as actual usage is the sum of the measurement and whatever the PVs can supply when the demand is made. Since we habitually turn on the washing machine and toaster at noon, actual usage may be much higher than the plot. Similarly, it is not possible to measure with precision the surplus power we feed into the grid which attracts another modest payment.

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