Demand
Base household use is increased by EV mileage, heat-pump electricity and electric water heating. EV demand assumes 0.28 kWh per mile and is adjusted for charging time.
Independent UK planning tool
How the calculator works
This page sets out the calculator model in plain language so you can challenge each assumption. It covers demand, solar generation, direct use, solar and off-peak battery charging, export income, estimated system cost and payback. The model is deliberately simplified: it cannot see your roof, wiring, local network constraints, future tariffs or actual half-hourly meter data. Treat its result as a consistent comparison between scenarios, then replace working assumptions with an installer’s site-specific evidence.
Calculation methodology
The model, in plain English.
Published so you can challenge the assumptions rather than accepting a black-box answer. Last updated 27 September 2026.
Solar generation
Array size is capped by the maximum panel count. Regional yield is adjusted for weighted roof orientation, pitch and shading. Annual degradation affects the longer-term payback model.
Direct use
Weekday and weekend occupancy are weighted across seven days and adjusted by the selected half-hourly profile. Direct consumption cannot exceed household demand or generation.
Battery operation
Solar and overnight grid charge are capped at capacity × 365 × an 85% cycle factor. Overnight value is day rate × efficiency minus night rate. Low cycles or utilisation trigger an oversizing warning.
Annual benefit
Direct use and battery discharge avoid import cost; remaining solar earns the selected export income.
Payback
Nominal payback divides system cost by first-year benefit. The adjusted result accumulates benefits with price inflation and panel degradation until the cost is recovered.
Representative profiles used for testing
These profiles are regression-tested whenever the calculation code changes. They are worked examples, not promises of performance.
Heat-pump screening bands
The derived bands are flat 2,100 kWh, small house 3,250 kWh, medium house 4,200 kWh and large house 4,800 kWh of annual heat-pump electricity. They combine published floor-area reference points (49, 75, 96 and 110 m²), the Household Electricity Survey’s 130 kWh/m² primary-electric-heating figure and the government explanation that a well-designed heat pump can deliver about three units of heat per unit of electricity. Values are rounded screening proxies, not official heat-pump averages; replace them with a room-by-room heat-loss calculation and installer forecast.
ONS floor-area data · Household Electricity Survey · UK Government heat-pump guidance
The 85% cycle cap is a conservative model assumption below one full cycle per day. For context, Enphase publishes a 15-year or 6,000-cycle warranty for its IQ Battery 5P; the calculator does not treat that product warranty as a prediction for every battery.