It’s obvious that solar PV systems need as much sunlight as possible – so the ideal is to position them at about 37 degrees to the horizontal (in the UK) and facing due South. Even better, if you can afford one, are tracker systems that ensure the modules follow the sun through the sky – although watch the cost/benefit of this.

What people don’t always realise is that hard shading (for example from chimneys or trees) falling across the panels can reduce their output by up to 90 per cent - even if the shade only obscures a small part of the system. This can be a little difficult to predict, because where the sun is in the sky varies through the year and with time of day, and hard shadows move accordingly.

On large schemes engineers will model shading using computer simulation and CAD tools. On smaller schemes it is more usually done by guesswork and experience. This means that if you own and live in the property where the system is going to be fitted it’s worth watching what happens to shadows on your roof for a time before you finalise the location of your modules – you may well be the best person to know where the least shaded area will be.

In the US, formal shading reports are required before government grants are provided for solar systems. In response simple portable tools have been developed to measure onsite shading before systems are bought – it is entirely possible to use geometry and image processing to predict sunpaths and shading for any site.  We have brought one of these tools (SunEye) to the UK recently. It will be interesting to see how widely the industry takes up this approach.

Photo by L@ura

About the author: Graham Eastwick is a director of Encraft, and manages renewable energy installations for home owners, community organisations and small businesses across the UK.

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