The UK rule of thumb
In the UK, a solar panel system generates roughly 850–1,100 kWh per kWp (kilowatt-peak) installed per year under average conditions. This figure — called the specific yield — varies by location, orientation, and shading.
A typical UK residential installation is 3.5–4 kWp (10–12 panels at 350–400W each). At 850 kWh/kWp in a cloudy northern location, that is around 3,000 kWh/year. At 1,100 kWh/kWp in the sunnier South East, the same system produces closer to 4,000 kWh/year.
Location matters: UK regional variation
The South West (Cornwall, Devon) and South East (Kent, Sussex) receive the most solar irradiance in the UK — typically 1,000–1,100 peak sun hours per year. Scotland and the North of England receive 750–900, with the Highlands at the lower end.
For a more precise estimate, the EU's PVGIS tool (free, online) allows you to input your exact postcode, panel angle, and orientation and returns a month-by-month generation estimate. This is the same data set that most solar installers use for their performance guarantees.
The difference between a south-facing and a north-facing roof at the same UK location can be 40–50% in annual generation. East- and west-facing roofs typically achieve 80–85% of the output of an equivalent south-facing installation.
Performance losses: where your output goes
No solar system delivers its nameplate rated output under real-world conditions. The main loss factors are: temperature (solar cells are rated at 25°C; efficiency drops above this, which is why hot sunny days are not always the best generating days); cable and inverter losses (typically 5–10% combined); shading from chimneys, trees, or neighbouring buildings; and soiling — dust, pollen, bird droppings, and mineral residue on the glass.
Soiling losses in the UK average 2–5% annually for a typical residential installation that receives no cleaning. In areas with high pollen, heavy traffic, or persistent bird activity, losses of 10–20% have been measured before an overdue clean.
The practical upshot is that a clean system in a good UK location should hit the PVGIS estimate within 5–10%. A system consistently reading 15% or more below that figure, with no obvious technical fault, almost certainly has a soiling problem.
Reading your inverter: what the numbers mean
Your inverter display or monitoring app will show several figures. Current power (in kW or W) is the instantaneous output right now. Today’s generation (in kWh) is your total for the day so far. Total yield is the cumulative generation since installation.
For assessing whether your system is performing correctly, today’s generation on a clear sunny day in June or July is the most useful number. A well-oriented, unshaded, clean 4 kWp system in the South of England should generate 20–28 kWh on a sunny midsummer day. The same system in the North of England, 18–24 kWh. Consistently achieving significantly less than these benchmarks on clear days is a signal worth investigating.
The cleaning payback calculation
If soiling is reducing your output by 10% and your system generates 3,500 kWh/year, you are losing 350 kWh annually. At current UK average export rates of around 15p/kWh (or self-consumption value at 25–30p/kWh if you are using the electricity directly), that is £52–£105 of lost value per year.
A professional clean for a 10–14 panel system typically costs £60–£100. In most cases the clean pays for itself in one to three months of recovered output. For systems with confirmed bird dropping issues, the payback can be even faster.