Estimating solar panel output in the UK: do the maths yourself

How much electricity your panels generate decides how quickly they pay for themselves. A good installer gives you a generation estimate, but a few numbers let you sanity-check it yourself.

The rule of thumb: kWh per kWp

Output is expressed as kWh per kWp per year. A useful national benchmark comes from the government’s energy statistics: the UK load factor for solar PV was 11.1% in 2025, up from 9.2% in 2024 ¹. Multiply by the 8,760 hours in a year and that is roughly:

Two lessons from that table. First, the weather matters: output can swing by a fifth from one year to the next. Second, these are averages over all UK solar, including poorly oriented and shaded systems; a clear south-facing roof in southern England will generally beat them, a shaded roof in Scotland will not.

Worked example at 900 kWh per kWp

For comparison, the Energy Saving Trust puts the average home system at around 4.5 kWp ².

Orientation and pitch

The Energy Saving Trust’s guidance ²:

• An unshaded, south-facing roof is ideal.
• East- or west-facing systems generate roughly 15–20% less, but spread production across the morning or evening – useful for self-consumption.
• North-facing installations are not recommended.
• On roofs pitched at 15° or more, rain keeps panels reasonably clean in the UK.

Flat roofs usually get tilted frames (south or east–west). Panels laid completely flat collect more dirt and produce less.

Shading: small cause, big effect

Panels are usually wired in series (strings). A shadow from a chimney, dormer or tree on one panel can, without countermeasures, drag down the whole string. The Energy Saving Trust notes that nearby buildings, trees or chimneys can hurt performance and that optimisers can reduce the effect ². Options:

• put panels only where there is no shade – the cheapest fix;
• an inverter with several MPPT inputs, so each roof face is handled separately; see our inverters;
• optimisers or microinverters per panel where shade moves across the roof.

Temperature: heat costs output

Even in the UK, panels reach 50–60 °C on a bright summer day, and lose power according to their temperature coefficient. In our database (1,000 panels from the Californian CEC list ³) the median for crystalline panels is −0.448%/°C, and the best crystalline panel manages −0.36%/°C. Airflow behind the panels helps. See the panels that cope best with heat.

Do it yourself with PVGIS

PVGIS is a free tool from the European Commission’s Joint Research Centre. It gives solar radiation and PV performance data for almost any location in the world, using hourly weather data plus terrain and horizon data ⁴ – and it covers the UK. The newest version, PVGIS 6, is still in beta ⁴.

How to use it:

1. Find your address on the map.
2. Choose Grid connected and enter the peak power in kWp (number of panels × watts ÷ 1,000).
3. Enter the roof slope and azimuth (0 = south, −90 = east, 90 = west). Calculate two roof faces separately.
4. Leave system losses (cables, inverter, dirt) at the default unless your installer gives a justified alternative.
5. You get annual and monthly output. Compare it with the estimate in your quote.

PVGIS does not know about your own chimney or your neighbour’s tree, so knock something off for local shading.

Checking your installer’s estimate

1. Does the quote state a yield per kWp? If it is well above what PVGIS gives for your roof, ask why.
2. Have shading and the real orientation and pitch of each roof face been included?
3. What self-consumption share is assumed, and is it realistic for your household?
4. Is panel degradation over the years included in long-term savings figures?

Also remember the seasons: most generation comes between April and September, and December and January add only a small fraction. A system that covers your annual use on paper will over-produce in summer and fall short in winter.

From kWh to savings

Split your expected generation into self-consumption (saves the full import price) and export (paid at your Smart Export Guarantee rate). Because a used unit is usually worth far more than an exported one, the timing of your consumption matters as much as the total. Compare panels on measured values in our solar panel database.