What a Tunbridge Wells commercial roof actually yields: PVGIS by pitch and month
Published 2026-09-26 · 5 minute read · By Commercial Solar Finance editorial team
The research question behind Kent enquiries: installers quote one annual number, lenders model another. Here is the public one, pulled from PVGIS on 26 September 2026 for a TN1 postcode, and what the pitch and the month do to it.
PVGIS is the European Commission's photovoltaic performance tool, built on satellite irradiance (SARAH3, 2005–2023). Fed a Tunbridge Wells location, one kilowatt-peak of unshaded south-facing panels at a 35° pitch and 14% system losses, it returns 1,062 kWh a year. That is the headline figure worth remembering for commercial solar in Royal Tunbridge Wells — and the first thing to adjust.
Pitch: the number a warehouse actually gets
| Setting (south-facing, unshaded) | Annual yield | Versus 35° |
|---|---|---|
| 35° pitched roof | 1,062 kWh per kWp | — |
| 10° low-pitch or flat roof with tilted modules | 964 kWh per kWp | −9% |
Most commercial roofs in Kent are not 35° slate. North Farm units and the sheds along the A21 are low-pitch steel, and modules on them sit near 10°, so 964 is the honest planning figure. East–west laid arrays, common where roof space is the constraint, lose more again in winter because neither face sees the low sun for long.
Month: a third as much in December as June
The same PVGIS run gives 130.9 kWh per kWp for June and 36.7 for December at 35°. A 100kWp array produces about 13,100 kWh in June and 3,700 kWh in December. For the finance model that means two things: the export line is a summer phenomenon, and a battery bought to shift solar into winter evenings will mostly be charged from the tariff between November and February. Sizing storage to summer surplus over-buys.
What the yield does to the four numbers a lender looks at
- Generation: 100kWp × 964 kWh = about 96,400 kWh a year at 10°, not the 106,000 a 35° figure implies.
- Avoided import: at 80% self-consumption and 25p/kWh that is about £19,300 a year; every 5 percentage points of self-consumption is worth roughly £1,200.
- Export: the remaining 20% at Octopus's 12p/kWh Outgoing rate is about £2,300 — useful, never decisive.
- Payback: on £85,000 installed, roughly 3.9 years simple before tax at 10° against 3.6 at 35°; the Annual Investment Allowance relieves the whole cost in year one either way.
The borough has already installed 3,330 photovoltaic sites and 48.2 MW of capacity (DESNZ, end 2024), so a Tunbridge Wells lender has seen the numbers before; what they want is the pitch-corrected yield and half-hourly demand. Both are on the Tunbridge Wells commercial solar finance page, with the finance routes compared.
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