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Solar ROI methodology

How the calculator estimates UK solar panel payback, battery savings, electricity usage, export income and confidence ranges.

Model inputs and versioning

The current calculator version is 2026-05-electricity-model-v1-solar-lookup-v1. The model starts with property type, bedroom count, postcode area and main heating type. The personalised journey can then refine the estimate with occupants, floor area, EPC band, EV usage, roof direction and exact annual electricity consumption.

Solar generation model

Annual generation is estimated as solar size in kWp multiplied by a UK regional kWh-per-kWp lookup, roof-orientation factor and shading factor. The lookup is based on conservative regional priors, with south-facing output ranging from 850 kWh/kWp in Scotland to 1,060 kWh/kWp in the South West.

Electricity consumption model

Electricity consumption starts from property type and bedroom archetypes, then adjusts for postcode area, floor area, occupancy, EPC band, heating type, electric hot water, cooking and EV mileage. Known annual consumption overrides the model for ROI calculations while preserving the model-only baseline.

Battery storage model

Battery storage is modelled through usable shifting rather than perfect storage. Daily cycling is capped through an annual shift limit of battery capacity multiplied by 240 cycles and no more than 33% of annual household consumption. Solar self-consumption is increased when battery storage is present.

Payback and ROI formulas

Annual benefit equals import savings plus export income plus battery tariff-shifting savings. Payback equals upfront cost divided by annual benefit. Lifetime benefit compounds annual benefit over the model horizon, and net profit equals lifetime benefit minus upfront cost.

Core formulas

Annual solar generation = system size kWp x regional yield x roof-orientation factor x shading factor.

Annual import savings = self-consumed solar kWh x import tariff. Export income = exported solar kWh x export tariff. Battery shifting savings = shifted kWh x the positive difference between import and off-peak rates.

Simple payback = upfront system cost / annual benefit. Net profit = compounded lifetime benefit - upfront system cost.

Default financial assumptions

Assumption	Default value
Import tariff	28p/kWh
Export tariff	15p/kWh
Off-peak tariff	12p/kWh
Standing charge	55p/day
Daytime electricity use	45%
Annual energy price rise	4%
Model horizon	20 years
Default shading	Light

Regional solar generation lookup

The lookup is a conservative proxy for PVGIS-style irradiance assumptions. It keeps the public calculator fast while making regional and orientation differences visible.

Region	South	South-west	West
South West	1060 kWh/kWp	1007 kWh/kWp	890 kWh/kWp
South East	1020 kWh/kWp	969 kWh/kWp	857 kWh/kWp
London	1000 kWh/kWp	950 kWh/kWp	840 kWh/kWp
East of England	990 kWh/kWp	941 kWh/kWp	832 kWh/kWp
East Midlands	970 kWh/kWp	922 kWh/kWp	815 kWh/kWp
West Midlands	950 kWh/kWp	903 kWh/kWp	798 kWh/kWp
Yorkshire and The Humber	930 kWh/kWp	884 kWh/kWp	781 kWh/kWp
North West	905 kWh/kWp	860 kWh/kWp	760 kWh/kWp
North East	890 kWh/kWp	846 kWh/kWp	748 kWh/kWp
Wales	965 kWh/kWp	917 kWh/kWp	811 kWh/kWp
Scotland	850 kWh/kWp	808 kWh/kWp	714 kWh/kWp
Northern Ireland	870 kWh/kWp	827 kWh/kWp	731 kWh/kWp
UK Average	955 kWh/kWp	907 kWh/kWp	802 kWh/kWp

System size and cost assumptions

Property type	Solar size	Roof area	Solar cost model	Battery cost model
Detached	6.2 kWp	31 m2	GBP 1850 + GBP 1050/kWp	GBP 2050 + GBP 420/kWh
Semi-detached	4.6 kWp	23 m2	GBP 2100 + GBP 1100/kWp	GBP 2150 + GBP 430/kWh
Terraced	3.3 kWp	17 m2	GBP 2300 + GBP 1160/kWp	GBP 2250 + GBP 440/kWh
Flat	2.2 kWp	11 m2	GBP 2550 + GBP 1225/kWp	GBP 2400 + GBP 455/kWh

Run your estimate

See how the assumptions change your result

Start with the quick calculator and refine the estimate after sign-up with exact consumption and property details.

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