The inputs that matter
Roof aspect and pitch, annual irradiance for your postcode, shading factor (the most-fudged input), system size in kWp, expected self-consumption %, current import price, current export rate, and battery capacity if specified.
Where calculators mislead
Many ignore shading, assume optimistic self-consumption (60%+ without a battery is rare), and project import prices upward indefinitely. Our quotes use conservative real-world assumptions — and we'll show you the spreadsheet.
The base formula in plain English
Annual generation (kWh) = system size (kWp) × site-specific yield (kWh/kWp/year, derived from PVGIS). Annual savings (£) = (self-consumed kWh × import price) + (exported kWh × export rate). Payback (years) = installed cost (£) ÷ annual savings (£).
Every reputable calculator runs that same formula; what differs is the realism of the inputs. A calculator that quotes you 4-year payback is either using inflated price assumptions or unrealistic self-consumption rates.
How we model self-consumption honestly
Without a battery: 25-35% for a household out at work weekdays, 35-50% for a household with someone home daytime, 40-55% if you also have an EV charged at home during daylight hours.
With a battery: 70-90% depending on capacity match and tariff. We use 80% as a default conservative number; many homes do better in practice once they tune their settings over the first three months.
What irradiance number should I use?
PVGIS (European Commission) is the industry-standard data source. For the South East, typical numbers are 1,000-1,100 kWh/kWp/year on south-facing pitched roofs at 30-40°. East/west aspects lose 12-18%. North-facing pitches lose 25-32%. Pitch above 50° or below 15° loses a further 3-7%.
Should I model electricity price inflation?
We don't, by default. Conservative payback at today's prices is the cleanest number to compare quotes against. If you want to see the picture with inflation, we'll run scenarios at 0%, 3% and 5% — but we won't use those scenarios to justify a different quote.
Sanity checks on any quote
Annual generation should be roughly 950-1,100 × system size in kWp. Annual saving should be roughly £150-£200 per kWp on solar-only, £220-£320 per kWp with a well-sized battery. Payback should fall in 7-12 years. Anything noticeably outside those ranges is worth questioning before signing.