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.
Use the official MCS register to find certified solar and battery installers in your area. MCS certification is required for SEG payments.
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