PVGIS yields for farm roofs by UK region (2026)

Every quote for solar panels for farm buildings rests on one number that most farms never check: the yield assumption, in kilowatt-hours per kilowatt-peak per year. Multiply it by the array size and you have the generation forecast; multiply that by the self-consumption ratio and the import price and you have the saving. Get the yield wrong by 15% and the payback moves by a year. This post sets out what the European Commission’s PVGIS tool — the free, public solar-resource database that serious installers use — gives for a typical low-pitch farm roof across the UK, and how to use it to check a quote.

The numbers, for a 10° pitch facing south

Portal-frame livestock sheds, grain stores and cubicle housing are mostly low-pitch roofs, often around 10–15°. For a south-facing array at a 10° pitch, PVGIS (version 5.3, crystalline silicon, default system losses) gives roughly:

The spread between the top and bottom of that table is more than 20%. The same 100 kW array on the same building type generates about 77,000 kWh a year in Glasgow and about 95,000 kWh in Cardiff. A national “850–1,100 kWh per kWp” range is not wrong, but it hides the fact that where your farm is decides which end of it you get.

Pitch and orientation move it again

The 10° figures are close to a floor for a south-facing roof. A steeper south-facing pitch — a traditional barn at 30–35° — does better; for Scotland at a 35° south-facing pitch PVGIS gives roughly 810–938 kWh per kWp depending on the site, above the 772 low-pitch Glasgow figure. An east–west roof, which is what most portal frames are when the ridge runs north–south, gives less than a south-facing one at the same pitch but spreads generation across the day, which can suit a farm load. Shading from a taller adjacent building, a tree line or a silo takes a further slice, and PVGIS does not know about that unless the installer models it.

How to read the yield line in a quote

1. Find it. A proper proposal states the yield assumption explicitly. If it only gives you a generation figure in kWh with no yield behind it, ask for the yield and the orientation, pitch and location it came from.
2. Check it against PVGIS. Put your postcode, pitch and azimuth into the PVGIS tool yourself. It takes five minutes. If the quote’s figure is well above what PVGIS gives for your roof, ask why — occasionally there is a good answer (a newer module efficiency assumption, a lower loss factor), often there is not.
3. Treat anything above about 1,000 kWh per kWp in the UK as a figure to challenge. It is achievable on the best south-facing roofs in the south-west; it is not a default.
4. Check the losses. PVGIS defaults to 14% system losses. A quote using a much lower figure without explaining the cable runs, the inverter efficiency and the soiling assumption is optimistic.
5. Then look at self-consumption. Yield tells you what the roof makes; the half-hourly meter data tells you what the farm can use. On a dairy parlour most of it; on a grain store outside harvest, much less. The two numbers together, not either alone, set the payback.

Why installers argue about this

A higher yield makes every quote look better, and because it sits behind a single line item most buyers do not question it. The honest installers put the PVGIS screenshot in the proposal. The agricultural solar panel installers guide covers the other checks — registers, G99 sign-off, references — and the payback calculator lets you run your own yield and meter data through the same arithmetic before anyone quotes.

The short version

For a low-pitch south-facing farm roof, expect roughly 770 kWh per kWp in central Scotland, 810 in the north-west of England, 920 in London and 950 in south Wales, more on a steeper south pitch and less on an east–west roof. Check the figure in every quote against PVGIS for your own postcode, and read it alongside the self-consumption ratio from your meter data. Those two numbers are the project; the panel brand is not.