Two ways a home battery makes you money: storing your own solar to avoid expensive imports, and charging from cheap overnight electricity to displace peak-rate use. We model both — and tell you how long the maths takes to work.
Solar self-consumption uplift. Without a battery, ~40% of your solar gets used in the home and the rest exports at the SEG rate. A battery raises self-consumption progressively up to ~80% at 15kWh capacity. Each extra kWh self-consumed earns the difference between your import rate and your SEG rate — typically 17–18p per kWh.
Time-of-use arbitrage. On Octopus Go (~8.5p off-peak), Cosy (variable rate windows), or Intelligent Octopus, your battery can charge cheaply overnight and discharge during peak hours. We assume ~280 useful cycles per year (200 if you also have solar — there's some overlap), 90% round-trip efficiency, and that 55% of your usage falls in peak hours.
Battery cost reflects 2026 UK installed prices: roughly £350 per kWh of capacity plus £2,000 base for inverter and installation.
12-year horizon matches typical battery warranties. We apply 1% capacity loss per year. Many batteries outlast their warranty but warranty period is the conservative assumption.
Real performance varies enormously with usage pattern — a household at home all day captures less TOU benefit than one out at work and home in the evening. These numbers are estimates.