Battery Sizing Guide

What size solar battery do I need?

Solar battery sizing guide — how much capacity you actually need: SIZING TABLE (by household + solar system): 3kW solar, couple (out during day): 5kWh optimal. Captures 90% of available surplus. Cost: £3,000-£3,500. 4kW solar, family: 8-10kWh optimal. Captures 92% of surplus. Cost: £4,000-£5,000. 5kW solar, family + EV: 10-13.5kWh optimal. Covers evening + some overnight EV. Cost: £4,500-£6,000. 6kW+ solar, large home + EV + heat pump: 13.5-20kWh. Maximum self-consumption. Cost: £5,500-£8,000. DIMINISHING RETURNS: 5kWh captures 85% of surplus value. 10kWh captures 93%. 15kWh captures 96%. 20kWh captures 97%. The jump from 5→10kWh adds meaningful value. From 10→15kWh: marginal. From 15→20kWh: rarely justified. SMART TARIFF EXCEPTION: On Octopus Flux/Agile, larger batteries earn more through tariff arbitrage (buy cheap, sell dear). 10-13.5kWh is optimal for Flux users regardless of solar system size. Sources: Battery manufacturer data, PVGIS generation profiles, BEIS consumption patterns, Octopus Energy tariff rates.