Updated
Solar Panels Producing Less Than They Used To
The system still runs, but the numbers are well below what the same month used to give you.
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What's happening?
Falling output is the most reported solar problem and the one most often misdiagnosed, because British weather makes any two months incomparable. The only honest comparison is the same month in a previous year, ideally against the estimate in your original quote. Real causes fall into four groups. Shading, which changes over years as a tree grows or a neighbour extends. Soiling, where dirt, moss or lichen builds along the bottom edge of each panel where water sits. A partial electrical fault, where one string of panels has dropped out and the inverter carries on happily at half output. And genuine degradation, which is real but slow — panel manufacturers typically warrant a defined percentage of original output at twenty-five years, so a healthy array loses a fraction of a percent a year, not a third of its output in a season. A sudden step down points at a string or an inverter. A slow slide over years points at shading or soiling.
What usually causes it?
- A tree, hedge or neighbouring extension that has grown into the array's afternoon sun
- Moss, lichen or traffic film along the lower edge of the panels where water and dirt collect
- One string offline, so the inverter still reports but at roughly half the expected power
- A failed or degrading optimiser or micro-inverter on individual panels
- An inverter derating in hot weather because it is mounted in an unventilated loft
- Grid voltage running high, so the inverter trims output to stay within its limits
- A monitoring app comparing against the wrong system size or a different tariff period
Is it dangerous?
Lost output is not dangerous by itself. What matters is what caused it: a string that has dropped out means a DC connection somewhere has failed, and failed DC connections are the origin of most solar fires. If output fell suddenly, treat it as a fault to investigate rather than a seasonal dip — particularly if it is accompanied by any smell, discolouration or noise at the isolator or inverter.
Can I fix it myself?
Do the comparison properly first. Open your monitoring and put this month next to the same month last year and the year before, then against the annual estimate on your original quote. Walk outside at midday and look at the array for shadows, and look at the bottom edge of the panels for a green or black band. Check the inverter is showing all strings, not just running. Check nothing indoors has changed — a new export arrangement or a battery installed since can make self-consumption look like lost generation. Do not go on the roof.
When should you call a solar installer?
Call a solar installer if output dropped in a step rather than a slide, if the inverter shows one string at zero, if generation is well below the original estimate in a good month, or if the system has never matched what was quoted. An MCS certified installer can test the strings and compare against the design figures in your certificate.
What will the solar installer actually do?
- Compare measured generation against the design estimate on the MCS certificate for your system
- Test each string's open-circuit voltage and current to find one that has dropped out
- Check optimiser or micro-inverter reporting panel by panel where the system has them
- Inspect the array for shading, soiling and any physically damaged or delaminating panel
- Check inverter operating temperature and ventilation, and log grid voltage over a period
- Clean or replace failed DC connectors and re-test the string before signing it off
Frequently asked questions
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