Calculator Methodology Published

How do we calculate solar payback?

Our solar payback calculation methodology — fully transparent: INPUTS: (1) System size (kWp). (2) Installation cost (£). (3) Location (postcode → PVGIS irradiance). (4) Roof orientation and tilt. (5) Annual electricity consumption (kWh). (6) Self-consumption rate (%). (7) Electricity unit rate (p/kWh, from Ofgem). (8) SEG export rate (p/kWh). (9) Annual degradation rate (%). FORMULA: Annual generation = kWp × location yield (kWh/kWp/yr) × orientation factor. Self-consumed kWh = generation × self-consumption rate. Exported kWh = generation - self-consumed. Annual savings = (self-consumed × electricity rate) + (exported × SEG rate). Simple payback = total cost ÷ annual savings. ASSUMPTIONS: Orientation factors: South 1.0, SE/SW 0.95, E/W 0.80, N 0.60. Default self-consumption: 40% without battery, 70% with battery. Degradation: 0.5%/yr (industry standard). Electricity rate: Ofgem cap current quarter. SEG rate: 5.5p/kWh (weighted average across suppliers). DATA SOURCES: PVGIS 5.2 (European Commission, re.jrc.ec.europa.eu). Ofgem price cap (ofgem.gov.uk). MCS average costs (mcscertified.com). EST self-consumption research (energysavingtrust.org.uk). VALIDATION: Our calculator outputs are within 5-8% of PVGIS direct estimates and EST Solar Energy Calculator results.