Update of Wivey solar estimate

I have kindly been supplied with more data and details of another installation, so have updated my estimate of Wiveliscombe’s solar powered electricity.

The extra data has also allowed a slight change to the calculation method used, which has slightly lowered the outcome to the equivalent of 55% of domestic electricity we use being generated by solar power.

Panel orientation is now better taken into account. With data for more installations, I’ve calculated outputs (kWh) per annum by unit of generating capacity (Wp) for south-facing installations in Wivey, which averages 0.99 kWh pa per Wp of installed capacity (with the lowest south-facing array achieving 0.90 and the highest 1.07).

I have less data for installations not directly facing south, which includes systems facing both east and west. Based on this more limited data, it appears that panels facing mostly to east or west generate about 80% as much as those facing south and those facing south-east or south-west generate about 90% as much. The best installation of those facing east or west achieved 0.89 kWh pa per Wp and the lowest was 0.74.

Including the Sandys Moor solar farm, 97% of Wivey’s known solar panels face south, but excluding the solar farm, those on 42 roofs and two gardens show more variety, with 66% facing south.

The output by capacity (kWh pa per Wp) performance indicator is at the heart of my new calculation and has been used to make individual estimates of output for installations for which actual readings have not yet been supplied, which are all now shown at Wivey solar installations and output.

Any owners seeing this who have not supplied any annual readings to me, may like to compare my estimate of their solar generation to their actual figures, and, whether it proves a good guide or not, I would greatly appreciate receiving any more data that owners are willing to pass on (generation over calendar years is especially helpful and does not have to exactly match the year start and end if the dates that apply are also supplied).

The new estimate also uses a more recent figure for domestic electricity consumption in Taunton Deane. Official energy statistics for 2012 are the latest available and show our electricity use went up to 4,430 units (kWh) that year.

Dave M


2 thoughts on “Update of Wivey solar estimate

  1. The Microgeneration Certification Scheme (MCS) Guide to the Installation of Photovoltaic Systems (http://www.microgenerationcertification.org/images/PV%20Book%20ELECTRONIC.pdf) includes a standard estimation method (page 59) to predict annual solar generation. It also provides tables for zones throughout the UK with annual generation values that are equivalent to my output by capacity, which they express as kWh/kWp and call Kk values. For our zone (see pages 60-61 and page 101), these tables show a Kk value of 972 (or 0.97 kWh/Wp) for a south-facing installation on an optimal roof inclined at 38 degrees. The MCS values are used for standard output estimates supplied by installers.

    The MCS Kk value for our zone is a little lower than the value of 0.99 kWh/Wp, which I calculated from data supplied so far for south-facing installations in Wivey. This suggests Wivey installations are performing above those for our zone, which stretches from Dorset and into Gloucestershire. This may be due to our height above sea level or simply because our location is above average for solar irradiance within the zone (see page 56).

    It’s also possible that as more data becomes available (giving my calculation a smaller margin of error) that the Wivey average will move closer to the zone average, but currently my conclusion is that Wivey panels perform above average for our zone and so are likely to perform a bit better in practice than estimates supplied by installers.



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