If your solar generation looks lower than it used to, the first job is to work out whether that is normal seasonal variation or a genuine fault. Most "my solar isn’t working" calls turn out to be one of a handful of causes — and the fix is often cheaper than people fear, though sometimes the smarter money is an upgrade. Here is how to diagnose it.
Step 1: rule out the weather
UK solar output swings massively by season — a system generating 25 kWh on a clear June day might do 3 kWh on an overcast December one. That is normal. The test that cuts through it is the clear-day rule: on a bright, sunny day around midday, compare your current output against your install-year baseline (your monitoring app or your installer’s original estimate). A healthy system should get close to its rated output. A drop of more than ~10% on a clear day, not explained by season, is a real fault worth investigating.
Step 2: the common causes, ranked
In our experience the causes break down roughly like this:
- Inverter issues (most common). A tripped isolator, a recurring grid fault, or an ageing inverter at end-of-life. If output has dropped to zero on a sunny day, it is almost certainly the inverter — check the display for a code (our inverter fault codes guide explains what each means).
- Soiling. Bird muck, moss, pollen and lichen — worst near trees, farmland or the coast. A clean recovers 5–15% on a genuinely dirty array. Rain handles most light dust on its own.
- Shading. New growth from a neighbour’s tree, a new extension, or a TV aerial can shade a string and drag down disproportionate output. String inverters are especially sensitive.
- A failed or degrading panel. Micro-cracks, hot spots or a failed bypass diode on one panel can pull down a whole string. Thermal imaging finds these.
- Genuine age. Panels lose only ~0.4–0.5%/year, so after a decade a healthy panel still does ~95%. If your loss is far steeper, it is not normal degradation.
Step 3: the fix ladder
Work up the ladder from cheapest to highest-value:
1. Clean — if the array is visibly soiled and under ~8 years old, a clean may be all it needs. See solar panel cleaning cost. 2. Fix — replace a failed isolator, fix a connector, or swap a faulty panel/optimiser. 3. Retrofit — if the system is sound but you only use ~30% of what you generate, a battery retrofit does more for your bills than any repair, lifting self-consumption to 80%+. 4. Replace — if the inverter is at end-of-life and panels are ageing, a modern system from the same roof can produce far more. FiT-era panels (2010–2015) make 250–300W each; modern panels make 430–500W.
When repair is throwing good money after bad
The honest line we give customers: spending £150–£300 a year servicing a system that is past its best rarely pays. If your panels are 8+ years old and the inverter is failing, the spend is usually better put toward an upgrade or a full solar and battery installation that recovers the yield permanently and adds storage. We will quote both routes so you can decide.
Get a straight answer
Our free 5-point solar system health check measures your actual output against baseline, reads the inverter, and inspects panels and strings — for any brand. We tell you whether you need a clean, a fix, or an upgrade. Book online or call 0800 099 6606.