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Battery Storage Calculator — Home Battery Sizing
Calculate the optimal battery storage capacity for your home solar system. Compare savings with and without battery storage.
Average daily consumption
Installed or planned solar capacity
Your grid import rate (Ofgem cap ~26.11p from 1 Jul 2026)
Your SEG/export rate (UK 2026: ~4-12p/kWh)
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How We Calculate This
This calculator determines the optimal battery capacity by analysing how much solar energy you currently export and how a battery could capture that energy for later self-consumption.
How battery savings work
- Solar shifting: Store excess daytime solar generation for evening use, saving the difference between your import rate (Ofgem cap ~26.11p/kWh from 1 July 2026) and your export rate (typically 4-12p/kWh) per kWh
- Tariff arbitrage: With an off-peak tariff, charge from cheap overnight electricity (~7p/kWh) and discharge during peak periods
- Self-consumption boost: A battery sized to your daily solar surplus typically lifts self-consumption from 30-50% (solar only) to around 70%, and a larger battery can reach 80-90% — the calculator estimates this from your own system size and usage pattern
Sizing approach
The recommended battery size is based on capturing the majority of daily solar export while avoiding oversizing. Diminishing returns apply - the first few kWh of battery capacity provide the most benefit.
Frequently Asked Questions
Battery size depends on how much solar energy you currently export. A typical UK home with a 4 kWp solar system exports around 6-10 kWh per day in summer. A 5-10 kWh battery captures most of this surplus for evening use. Oversizing beyond your daily export provides diminishing returns unless you use off-peak tariff arbitrage.
At current UK electricity prices (the Ofgem cap is about 26.11p/kWh for direct debit from 1 July 2026) and battery costs (around £400/kWh installed), payback periods typically fall in the 7-12 year range, depending on your usage pattern, solar system size, export tariff, and whether you also use an off-peak tariff for arbitrage savings. Higher import prices, lower battery costs and a low export rate all shorten the payback. This tool estimates payback from your own figures.
Lithium Iron Phosphate (LFP) batteries offer more cycles (6,000+), longer lifespan, and better safety but are slightly larger and heavier. Lithium NMC batteries are more energy-dense (smaller) but have fewer cycles (4,000) and higher degradation. For home storage, LFP is increasingly preferred for its longevity.
Yes, batteries can be used for tariff arbitrage - charging from cheap off-peak electricity (around 7p/kWh on smart EV tariffs such as Intelligent Octopus Go) and discharging during peak periods (the cap rate is about 26.11p/kWh). This can save a few hundred pounds per year. However, the economics are best when combined with solar generation.
Most home batteries are warranted for 10 years or 6,000-10,000 cycles. LFP batteries typically retain 80%+ capacity after 10 years of daily cycling. Actual lifespan depends on charge/discharge cycles, depth of discharge, and temperature. Many batteries will last 12-15 years with moderate use.
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Last updated: June 2026
Verified against UK standards · estimates only, confirm with your supplier.