Solar Panel Payback Calculator

The calculator uses the MCS MIS 3002 methodology — the same framework used by certified installers to produce official solar generation estimates. It incorporates Ofgem Q1 2026 electricity rates, standard SEG export tariffs, panel degradation curves, and a Year 12 inverter replacement cost. Actual results will vary based on roof orientation, shading, local weather patterns, and your specific installation quote.

Kk is the MCS irradiance factor — the amount of solar energy available per kWp of installed capacity in your region, measured in kWh/kWp/year. The South West of England has the UK's highest Kk at approximately 1,060 kWh/kWp/year. Scotland Highland has approximately 785 kWh/kWp/year. The formula is: Annual generation (kWh) = System size (kWp) × Kk × Shading Factor × Performance Ratio.

Without battery storage, UK homeowners typically self-consume 35–42% of their solar generation, depending on occupancy and lifestyle. Working-from-home households can reach 45–50% without a battery. With a 9.5kWh battery system, self-consumption rises to 70–85% for most households. The calculator uses 38% without battery and 78% with battery as UK averages.

The SEG mandates that energy suppliers pay you for every unit you export. Standard switchable tariffs pay 15–16p/kWh. However, some time-of-use export tariffs (Octopus Flux, Intelligent Octopus) pay up to 31p/kWh during peak evening periods. Using a premium export tariff can reduce payback by 1–3 years compared to the standard rates modelled here.

Adding a battery typically increases total system cost by £4,000–£8,000 but raises annual benefit by £300–£450 for a 4kWp system. This generally extends payback by 2–4 years compared to a solar-only system, but significantly improves lifetime ROI and reduces dependence on grid electricity. If your main goal is financial payback speed, start without a battery. If energy independence and long-term returns matter more, battery storage is worthwhile.