More on factors affecting solar output

I’ve found detailed statistics on factors affecting solar output published by the Microgeneration Certification Scheme (MCS) in their 2012 Guide to the Installation of Photovoltaic Systems. This confirms “inclination, orientation and shading are the three main site factors that influence the performance of a PV system” (page 58); although, as they report, latitude is also significant, with the south having more solar irradiation than the north (see picture on page 56).

The MCS guide includes solar generation tables for zones throughout the UK (see pages 60-61 and page 101), which details how solar output varies with roof slope and orientation. [UPDATE: I’ve now prepared a version of this table showing how output varies for our zone – see section on Factors affecting solar output in Wivey (part-way down costs and benefits page).]

The peak roof angle for our zone is 38 degrees. Output is less than 1% lower at 30 degrees and only 2% lower at 25 degrees. For steeper roofs, output falls by only 0.5% at 45 degrees.

Orientation has a greater effect. Panels facing SE or SW have an output 5% lower than those facing South and those facing West or East have outputs that are 18% lower. This compares with my initial rough estimates of being 10% and 20% down respectively, which I have now updated.

For those wanting a simple guide to how solar output varies for a range of orientations and roof slopes, the MCS guide has a good indicative chart (figure 17) at the top of page 55, which shows variations for a mid UK point (the Wivey position will only vary slightly from this).

Dave M

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