What size home battery do I need?
Time-of-use tariff arbitrage works with or without solar. Tell us your daily usage and tariff — we size the battery and estimate annual savings from charging cheap and using at peak.
Your electricity use
Recommended battery
Updates as you change the inputs.
- Battery capacity
- — kWh
- Installed cost (range)
- £—–£—
- Annual savings
- £—
- Payback period
- — years
How the sizing works
Battery storage sizing for time-of-use tariffs is a different calculation from solar self-consumption sizing. With TOU arbitrage, you're charging the battery from cheap off-peak grid electricity (around 11–12p/kWh on Cosy or Agile) and using that stored energy to displace expensive peak-rate consumption (28–39p/kWh on Cosy, even higher on Agile spikes).
Sizing logic
For pure TOU arbitrage, you want a battery that covers your peak-hour consumption from off-peak storage. Most UK households use 30–40% of their daily electricity during the typical peak window (16:00–19:00). For a home using 10 kWh/day, peak-hour consumption is around 3–4 kWh — so a 5–7 kWh battery covers most of it with headroom for weekends and higher-use days.
With solar PV
When solar PV is in the mix, the battery captures excess generation during the day instead of exporting it at 8p/kWh (SEG). That stored solar is then used at peak hour, displacing 28–39p grid electricity. Effective price gap widens to 28–39p (no off-peak charging cost) and the battery cycles harder. Larger batteries (10–15 kWh) make sense alongside 4 kWp+ solar arrays.
Tariff matters
On a fixed tariff, batteries don't pay back from arbitrage — there's no price gap to exploit. Octopus Cosy is currently the most battery-friendly mass-market TOU tariff (large fixed peak/off-peak gap, predictable schedule). Octopus Agile can deliver bigger spikes but requires smarter scheduling to avoid charging during expensive grid hours.
What this calculator doesn't do
It assumes one full charge/discharge cycle per day at typical UK price gaps. Real-world performance varies with cycling depth, round-trip efficiency (typically 85–90% for lithium-ion), and seasonal price movements. A proper installer survey models your actual half-hourly usage profile against current tariff data.
Frequently asked questions
Can I get savings from a battery without solar?
Yes. On Octopus Cosy or Agile, the off-peak vs peak price gap (12p vs 28–39p) delivers savings from arbitrage alone. Without TOU pricing (fixed tariff), batteries don't pay back unless paired with solar.
How much does a home battery cost in the UK?
£500–£800 per kWh installed in 2026. A 10 kWh battery costs £5,000–£8,000 installed including inverter. VAT is 0% on residential battery installs until 2027.
What is the payback period?
On flat tariff with no solar: doesn't pay back. On Cosy/Agile without solar: 7–12 years. With solar paired: 3–7 years.