Size a home battery based on your electricity usage and solar panel output
Adding battery storage to a home solar panel system lets you use more of the electricity you generate rather than exporting surplus power to the grid at a low rate. With the gap between electricity import and export prices widening, a well-sized battery can meaningfully reduce your energy bills. In the UK, a typical 4 kWp solar system generates around 10 kWh per day on an annual average. Much of this generation happens during daytime hours when many households are at work. Without a battery, that surplus is exported to the grid at around 15p per kWh, while you buy it back in the evening at 24.5p per kWh. A battery bridges this gap by storing daytime surplus for evening use. Our solar battery storage calculator helps you find the right battery capacity for your system. Enter your daily electricity usage, solar panel size, and current tariff rates to see the recommended battery size, estimated daily and annual savings, and how long the battery takes to pay for itself.
To size a solar battery for your home: 1. Enter your daily electricity usage in kWh. The average UK household uses around 8.5 kWh per day. Check your smart meter or energy bill for your actual figure. 2. Enter your solar panel output in kWp (kilowatt-peak). This is the rated capacity of your solar array, typically between 3 and 6 kWp for residential installations. 3. Set the battery round-trip efficiency. Most lithium-ion batteries achieve 85-95% efficiency. The default of 90% is typical for current models like the Tesla Powerwall or GivEnergy. 4. Enter your electricity import rate in pence per kWh. The current Ofgem price cap rate is 24.5p/kWh. 5. Enter your export rate in pence per kWh. The Smart Export Guarantee (SEG) typically pays 12-15p/kWh. 6. Review the recommended battery size, daily and annual savings, and payback period based on a GBP 5,000 installation cost.
The solar battery storage calculator estimates optimal battery capacity and financial returns: Daily Solar Generation = Solar Output (kWp) x 2.5 kWh per kWp The 2.5 kWh per kWp figure is the UK annual daily average, accounting for seasonal variation, cloud cover, and panel orientation. Surplus Solar = Daily Solar Generation x 0.5 Approximately 50% of solar generation occurs while the household is not consuming it (daytime hours when residents are at work or out). Recommended Battery Size = Surplus Solar / Battery Efficiency This ensures the battery can capture and store the full daytime surplus. Usable Surplus = minimum of (Surplus x Efficiency, Daily Usage x 0.5) The usable surplus is capped at half your daily usage since you still consume some grid electricity during the day. Daily Savings = Usable Surplus x (Electricity Rate - Export Rate) / 100 You save the difference between what you would have paid to import and what you would have earned by exporting. Annual Savings = Daily Savings x 365 Payback Period = GBP 5,000 / Annual Savings
Inputs: Daily usage: 8.5 kWh. Solar output: 4 kWp. Efficiency: 90%. Import rate: 24.5p. Export rate: 15p.
Inputs: Daily usage: 12 kWh. Solar output: 6 kWp. Efficiency: 90%. Import rate: 24.5p. Export rate: 15p.
Inputs: Daily usage: 7 kWh. Solar output: 3 kWp. Efficiency: 85%. Import rate: 30p. Export rate: 12p.