Shading matters more on British roofs than almost anywhere else, because our houses are close together, our roofs are small and complicated, and mature trees are everywhere.
Why one panel affects the others
On a conventional system, panels are wired in series into a string. Current flows through each panel in turn, so the string is limited by its weakest panel.
A shaded panel does not just lose its own output. It restricts the flow through the whole string.
Modern panels include bypass diodes that reduce this by letting current route around a shaded section, which helps considerably. But the underlying effect is still there, and it is why shading costs more than the shaded area suggests.
What causes shading
Permanent:
- Chimneys, particularly on the south side
- Dormer windows and roof extensions
- Aerials and satellite dishes
- Neighbouring buildings
- Nearby trees
Seasonal and changing:
- Deciduous trees, which shade in summer and not in winter
- Low winter sun, which casts much longer shadows than summer sun
- Trees that will be taller in five years
That last one catches people out. Assess the trees you will have, not the trees you have now.
How to assess it yourself
Before your first appointment, look at the roof at three times on a clear day: mid-morning, midday and mid-afternoon. Photograph anything casting a shadow.
Remember that the sun sits much lower in winter, so a shadow that clears the roof in June may cover it in December. Anything to the south of the house is the main concern.
Take the photographs to every appointment. It makes the estimates comparable and it tells you quickly which installers are paying attention.
What can be done
Layout The simplest answer. Move panels away from the shaded area, or group the shaded ones onto their own string so they do not affect the rest.
Optimisers Small devices on each panel working with a string inverter. Each panel’s output is conditioned individually so a shaded panel does not hold back the others.
Microinverters A separate inverter per panel. Each is fully independent.
Both add cost. Both are worth it where shading is real. See microinverters and optimisers.
Back contact panels also handle partial shading somewhat better than conventional designs, which is worth knowing given how common shading is here. See panel technologies.
Removing or reducing the obstruction Sometimes the answer is pruning a tree. Check whether it has a tree preservation order or sits in a conservation area first.
What to insist on from an installer
- That they identify the shading, rather than you having to raise it
- That the generation estimate includes a shading factor, and that they can tell you what it is
- That the proposed layout takes it into account
- That any optimisers or microinverters proposed are justified by the actual shading, not fitted as standard
An installer who has not mentioned shading on a roof with a chimney has not produced a reliable estimate. That is the single most useful test of a solar quote.
Does shading make it not worth doing?
Usually not. Partial shading on part of a roof, properly designed around, reduces output somewhat rather than ruining the case.
Heavy shading across most of the roof for most of the day is different, and an honest installer will tell you. If one says the shading is fine and the other two say it is a problem, weigh that carefully.