Maintenance and fault-finding
How Solar Panels Perform in Winter in the UK
Solar panels keep generating through winter, but output falls sharply compared with summer. The cause is daylight hours and sun angle, not temperature. Knowing what normal winter numbers look like means you can spot a genuine fault rather than worrying about perfectly healthy seasonal behaviour.
Helpful video reference. Gary Does Solar's "365 Day Solar Generation: How Solar can perform in Winter (UK)" uses real UK generation data across a full year to show how output changes month by month, and what product choices affect winter yield. Gary Does Solar is an independent UK solar education channel.
1. What actually causes the winter drop
The most common misconception about UK winter solar performance is that the cold is to blame. It isn't. Solar cells actually run marginally more efficiently in cold weather, because lower temperatures reduce the electrical resistance of the semiconductor materials inside the panel. What causes the winter generation drop is entirely a geometry and daylight problem.
In late December, a UK location at the latitude of Kent has roughly 8 hours between sunrise and sunset, compared with around 16 hours in June. More importantly, the sun rises to only about 15 to 18 degrees above the horizon at its peak, compared with over 60 degrees in midsummer. That low angle means sunlight hits your panels at a shallow angle, reducing the effective area capturing energy. The result is roughly 20 to 30% of summer output across December and January, even on clear days.
2. What healthy winter numbers look like
A well-sited 4kWp south-facing system in East Kent should generate roughly 80 to 120 kWh across the whole of December. That is roughly 3 to 4 kWh on a good clear day, and under 1 kWh on a heavy overcast day. By contrast, June regularly produces 400 to 500 kWh across the month from the same system.
Rough winter output by system size (Kent, December)
- 3kWp system: 60 to 90 kWh for the month, roughly 2 to 3 kWh on a clear day.
- 4kWp system: 80 to 120 kWh for the month, roughly 3 to 4 kWh on a clear day.
- 6kWp system: 120 to 180 kWh for the month, roughly 4 to 6 kWh on a clear day.
These are estimates for a south-facing roof with no significant shading. East or west orientations will produce less. Your MCS certificate should include a predicted annual yield: divide by 12 and halve it for a rough winter monthly target.
3. Using your inverter monitoring app
Every modern grid-tie inverter comes with a monitoring portal or mobile app. This is your most useful tool for understanding winter performance. On a clear winter morning, log in and check the generation curve for the previous day: it should show a gradual rise from sunrise, a broad plateau in the early afternoon (lower and flatter than a summer peak), and a drop to zero by dusk.
If the curve shows a flat line at zero on a day that was clearly bright and sunny, that is a fault signal, not winter behaviour. If it shows a low but rising and falling bell curve, even if the peak is much lower than you'd like, the system is working as expected.
Most apps let you compare the current month with the same month last year. This is a good check if you suspect underperformance: a 10 to 15% year-on-year variation is normal depending on weather. A 30 to 40% drop compared with the same winter period, without any obvious shading change, is worth investigating.
4. When to call for a check
Contact your installer or a qualified solar service engineer if you notice any of the following:
Zero generation for two or more consecutive days that were visibly bright and sunny. A persistent fault or error code on the inverter display or in the app (most codes are listed in the inverter manual; some need a qualified engineer to reset). Generation that is more than 20% below the predicted annual yield shown on your MCS installation certificate, once you have ruled out an unusually poor weather year. A burning smell or visible damage near the inverter, DC isolator, or consumer unit connection.
Do not open the inverter casing or tamper with the DC isolators yourself. The DC side of a solar system carries high voltage even when the AC isolator is off, because the panels continue generating whenever there is daylight. Any electrical work on the system requires a qualified person.
Frequently Asked Questions
Do solar panels still work in the UK in winter?
Yes, UK solar panels generate electricity throughout winter, though output is typically 20 to 30% of what the same system produces in summer. The drop is caused by shorter days and a lower sun angle, not by temperature. A frosty, bright January day can produce a surprisingly decent generation total: it is the long dark Decembers and overcast cloud cover that limit the monthly figure, not the cold itself.
Does cold or frost damage solar panels?
No. Modern solar panels are designed to operate in a wide temperature range and are typically tested to -40°C. Frost on the panel surface clears quickly once daylight hits, and cold temperatures actually improve the electrical efficiency of photovoltaic cells slightly. Snow coverage does temporarily stop generation while panels are covered, but slides off quickly on most pitched roofs once the sun appears.
What should I do if my panels show zero generation on a bright winter day?
Open your inverter monitoring app and look for an error code. Common causes include a tripped AC isolator, a DC string fault, or an inverter issue. If the inverter shows no error and you cannot find a physical reason such as a tripped breaker, contact your installer or an MCS-certified service engineer. Do not attempt to open the inverter yourself: the DC side carries high voltage even when the AC supply is off.