Wivey homes are 59% solar powered

Wiveliscombe now has 13,479 solar panels – mostly installed over the last five years. They are on 70 local roofs, including both schools, there are two small ground-mounted installations and a solar farm hidden away on the edge of town.

Over the year, Wivey’s solar panels generate the equivalent of 59% of the domestic electricity used in the town council area.

Our solar output averages about 3,268 MWh or 3.27 million units every year, which saves about 1,750 tonnes of carbon dioxide emissions.

This calculation is based on actual generation data supplied for 93% of the panels in Wivey and estimates for the others. Average domestic electricity consumption (4,228 units) comes from official statistics for Taunton Deane published by the Government and, with new house building, I estimate there are now 1,310 homes in the Wiveliscombe town area.

Of course, in practice, Wivey homes will not use all of the solar power we generate. Some will be exported through the grid to be used by others. At times, it’s possible we will generate all (100%) of the domestic electricity we use and, at other times and at night, we will generate none. So the 59% calculation just reflects what we generate and use on average over the year. Nevertheless, it’s impressive.

The same contribution could be made UK-wide and generation and use could be matched through storage. There are many options and technologies are improving fast. Pumped storage (moving water to reservoirs at high points to generate hydro-electricity when needed) is long proven, with large pumped storage stations established in Wales in the 1960s. Battery storage is developing fast and renewable energy could be used to power the production of biogas and biofuels, which can then be used when needed.


Wivey solar panels rise to 13,184

There have been new solar electricity installations already this year in Ford Road, West Road and West Street and more existing ones been discovered in other parts of town, so we are now up to 52 known solar installations in the Wiveliscombe Town Council area.

There are now 994 solar PV panels on local roofs, two small ground-mounted arrays with 106 panels and 12,084 at Sandys Moor solar farm.

These generate the equivalent of 56% of the town council area’s domestic electricity over a year.

Factors affecting solar output

When comparing generation data kindly supplied by a number of solar panel owners in Wivey, one factor that stands out as making a difference to their output is orientation. Installations facing directly south produce more than others. This is expected and for installations in Wivey, panels facing east or west generate about 80% as much as those facing south and those facing south-east or south-west produce about 90% as much. [UPDATE: I have since established that panels facing east or west generate 82% as much and those facing SE or SW 95% as much.]

There are other factors that also affect solar electricity generation.

One of the most straight-forward guides to these is produced by Plug Into The Sun, an installer based in Cornwall. Their brief information sheet covers size of the array, different types of solar panel module (something on which installers should guide you according to your circumstances), orientation, roof slope or angle, latitude and shading.

Also informative is the Centre for Alternative Technology’s answer to ‘Is my home a good site for PV panels?’.

A roof slope of 30-45 degrees is best but from 20-50 degrees is still good if south-facing. On flat roofs, or those with just a slight incline, frames can be fitted to position panels at an optimum angle. [UPDATE: The optimum angle for south-facing panels in Wivey is 38 degrees but output should only be 2% lower at 25 degrees and just 0.5% lower at 45 degrees.]

Probably harder to address is shading, which is well-described in Plug Into The Sun’s guide. Signifiant shading is best avoided, but a small amount for a short period may not significantly affect output. This is shown by charts of solar power output throughout the day posted on Brendon Energy’s website for the Children’s Centre installation. On sunny days one tree shades some of the panels towards the end of the day, but overall output is only reduced slightly and largely confined to one of the three arrays at a time that make up the full installation. However, the effect could be greater on a smaller domestic system with a single array feeding into one inverter.

A time that all solar panel owners are likely to experience the effects of partial shading is when snow covers the panels. Before it fully melts, the snow is likely to start sliding off, but there may be a period when the snow just partially covers the lower part of the panels. Our experience of this was to find that no solar power was generated, even though the sun was shining and the snow just covered the lower part of the panels. The only consolation can be that this can be a good time for sledging and the snow doesn’t last too long, so generation soon returns to normal.

Good indications for the affects of varying roof angle and orientation are contained in solar calculator models, but it is interesting that data from Wivey panels confirms the affect of orientation on output. I’m sure it does for roof angle too but the slopes of most local roofs are OK and I’ve not (yet) collected data on this to compare!

Dave M

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

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