How a battery changes your solar economics
Without a battery, a typical UK solar system self-consumes about 45% of what it generates — the rest is exported at the SEG rate (~6p/kWh). A battery raises self-consumption to ~75%, meaning more of your solar power replaces grid electricity at the full import rate (~26.32p/kWh). That's nearly double the value per kWh.
How we calculate battery ROI
generation = system kW × sun hours
self-use without battery = generation × 45%
self-use with battery = generation × 75% (capped by battery capacity for daily cycling)
extra savings = extra self-use × import rate − lost SEG income
battery payback = battery cost ÷ extra annual savings
10-year profit = Σ extra savings (with 0.5% degradation + 3% inflation) − battery cost
When is a battery worth it?
- You're home in the evening — a battery shifts daytime solar into evening use, when you need power most. If you're out all day, half your solar is exported cheaply.
- You're on a time-of-use tariff — Octopus Agile / Go have cheap off-peak and expensive peak rates. A battery can charge at 9p and discharge at 30p+ — pure arbitrage profit, even without solar.
- You want backup power — a battery provides ~4–8 hours of outage protection. Not essential in the UK (grid is reliable), but valuable if you work from home or have medical equipment.
- You have a large solar system — 6 kW+ systems export a lot of surplus. A battery captures more of it. A 2 kW system barely benefits.
When is a battery NOT worth it?
- Small solar system (under 3 kW) — not enough surplus to fill the battery.
- You're on a flat tariff — no peak/off-peak spread to exploit.
- Your SEG rate is high — if your supplier pays 15p/kWh export, the gain from self-consuming instead of exporting is smaller.
- Battery payback over 15 years — most LiFePO4 batteries are warrantied for 10 years / 6,000 cycles. If payback exceeds the warranty, you're betting on the battery outlasting its guarantee.
Tesla Powerwall vs other batteries
The Tesla Powerwall 3 (13.5 kWh, ~£9,000) is the most recognised brand, but alternatives from Sigenergy, BYD, Sonnen, and SunGrow offer similar capacity at comparable or lower prices. Key specs to compare: usable capacity (kWh), depth of discharge (DoD), warranty (years + cycles), and round-trip efficiency. Most modern LiFePO4 batteries allow 90–100% DoD and last 6,000+ cycles.
Where the data comes from
- Import rate — Ofgem default-tariff cap, 26.32p/kWh (Q4 2026).
- SEG rate — typical 6p/kWh (supplier-dependent, 3–15p).
- Battery cost — ~£800/kWh for LiFePO4, ~£667/kWh for Powerwall 3 (installer data, 2026).
- Self-consumption — Energy Saving Trust: ~45% without battery, ~75% with (depends on usage patterns).
- Sun hours — PVGIS 5.2 regional values (European Commission), 2026.