Solar panels and shading: what it really costs you
Shading is the factor people most often underestimate, because the loss is bigger than it looks. A chimney shadow that covers a corner of one panel can cut the output of several, thanks to how panels are wired together. The good news is that modern electronics solve most of it, and a decent installer designs around shade rather than ignoring it. Here's what shade actually does, how it's fixed, and how to judge your own roof.
Why shade hurts more than its size
In a traditional setup, panels are wired in a chain called a string, and current flows through all of them in series. Like a hosepipe with a kink, the most restricted panel limits the flow for the whole chain. So if one panel is half-shaded, it doesn't just lose its own share, it drags down the panels linked to it. That's why a small, moving shadow can cost far more output than the shaded area alone, and why "it's only a little bit of shade" is misleading.
The fix: panel-level electronics
Two technologies break that chain effect by letting each panel work independently:
| Option | How it helps with shade |
|---|---|
| Power optimisers | A small device on each panel conditions its output so one shaded panel doesn't hold back the rest |
| Microinverters | Each panel has its own inverter, so panels are fully independent |
| Plain string inverter | Cheapest, but most affected by shade; fine for an unshaded roof |
On a roof with any meaningful shading, optimisers or microinverters usually recover most of the lost output and are well worth the extra cost. On a clean, unshaded roof they add expense for little gain. Matching the kit to the roof is exactly the kind of line item to check when you read a quote, covered in how to read a solar quote.
How to assess your own roof
Shade moves, so a single glance won't tell you much. Check it properly:
- Across the day. Look in the morning, at midday and in late afternoon. A shadow that's gone by 10am barely matters; one that sits on the roof through the productive midday hours matters a lot.
- Across the seasons. The winter sun is low and casts long shadows, so a chimney or tree that's clear in June can shade the array in December, the lean months covered in solar in a UK winter.
- The usual suspects. Chimneys, TV aerials, satellite dishes, vent pipes, taller neighbouring buildings, and trees.
A good installer does a formal shading analysis on survey, which feeds into the MCS performance estimate. It's one of the things worth asking about, as listed in questions to ask a solar installer.
Trees grow: think ahead
A sapling that casts no shadow today can shade your roof in ten years, well within the system's 25-year life. If a tree is on the line, factor in its growth, and remember you control your own trees but not a neighbour's. It's a small piece of forward planning that protects decades of output.
The detail most people miss
Shade and orientation work together, and shade often matters more. A south-facing roof shaded through the afternoon can produce less than an unshaded west roof, so the two have to be judged as a pair rather than separately, the point made in south vs east-west facing solar panels. If you take one thing from this: don't reject a roof for being the "wrong" direction before you've checked what's actually shading it.
Frequently asked questions
They still generate in light shade, just at reduced output. The problem is that in a traditional string, one shaded panel drags down the others, so partial shade costs more than it appears to.
It depends on how much of the roof is shaded and when, but losses are disproportionate to the shaded area in a plain string setup. Optimisers or microinverters recover much of it.
Yes. They let each panel work independently, so a shaded panel no longer holds back the rest of the string. Microinverters do the same job a different way.
Yes, and remember they grow. A tree that's clear of the roof now may shade it within the system's life, so factor in future growth when assessing shade.