Estimating Wivey’s solar powered electricity

Knowing how much solar electricity we generate from our panels in Wivey helps to indicate whether they are achieving something worthwhile and what contribution we are making to reducing our impact on the world.

So it is very encouraging to know that over a year my estimates show we generate the equivalent of 56% of our domestic electricity use from solar panels that have been installed over only the last four years.

Of course, this solar electricity is not all generated at the times we need to use it and so some, possibly even half or more, will be exported through the grid for others to use, but nevertheless we are making a good contribution in relation to our annual needs.

Some may be interested to know a little more about how I have made the calculation for the 56% estimate. The starting point is the list of panels shown on Wivey solar installations and output. Details for most of these have been obtained from the owners or, for the smaller installations, by simply counting the number of panels on the roofs. Some owners have told me their generating capacities, which mostly range from 210-250 kWp per panel. Panel potential or maximum capacities are steadily increasing over time. The latest installation on Wiveliscombe Community Centre in December 2013 has 265 kWp panels.

Where I do not know the panel power ratings, I have made assumptions for these based on typical values for the time at which they were installed.

A number of owners have also supplied me with meter readings for their actual solar generation, normally over their first year or for calendar years since installation. For Brendon Energy’s arrays on Wiveliscombe Children’s Centre, as well as taking monthly meter readings, I download stored output data direct from the three inverters, which, as well as daily and monthly totals, provides readings for every five minutes!

To provide an estimate of total solar generation from all Wivey’s panels, I have used the measured output from panels for which data has been supplied and projected this to apply for all the known panels, taking account of their expected power ratings.

Energy consumption data by district (4,261 kWh per household for Taunton Deane in 2011) is provided in official statistics. With 1,260 households in Wiveliscombe (Town Council area), this gives annual energy use of 5,368 MWh. Estimated solar generation is 2,992 MWh, which is 56% of total consumption.

This estimating method should provide a reasonable indication of solar electricity generation in Wivey. It will vary a bit from year to year with changing electricity use and solar output and, hopefully, will grow as more solar capacity is installed.

Please send further output data and installation details to me, which will enable these estimates to be further refined.

Dave M



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