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Solar Battery Storage Calculator
Size a home battery for your solar panel system based on daily electricity usage, daytime consumption patterns and tariff rates.
Check your smart meter or bill (UK avg ~8-10 kWh)
Your unit rate (Ofgem cap 1 Jul–30 Sep 2026: 26.11p/kWh)
Receive payment for exported solar electricity
SEG export rate (typical 4-15p/kWh)
Cheap overnight rate for grid charging
Average UK solar generation per kWp per day (default 2.5)
Fraction of evening usage to size battery for (default 0.8)
Installed cost per kWh (UK 2026 ~£650–900/kWh; default £700)
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How We Calculate This
This calculator estimates the optimal battery size for your solar panel system based on your electricity consumption patterns, system size and tariff rates.
How battery sizing works
The recommended battery size is based on the rule of thumb:
Battery size ≈ Daily usage × (1 - Daytime usage fraction) × 0.8
This is a common installer rule of thumb (not a published standard) — no British Standard prescribes home-battery sizing. It sizes the battery to cover roughly 80% of your evening and overnight electricity needs from stored solar. The result is then capped by your actual daily solar surplus and by your evening/overnight consumption — there's no point having a battery larger than what your panels can fill or larger than the energy you use after dark.
Solar generation
As a working average, UK solar panels generate around ~2.5 kWh per kWp per dayacross the year (more in summer, less in winter). A 4 kWp array therefore generates roughly 9–10 kWh per day on a well-sited, south-facing roof in southern England; the conservative UK average is nearer 3,400 kWh/year (~9.3 kWh/day), and northern or shaded roofs less again (MCS MIS 3002 irradiance / Energy Saving Trust). Adjust the per-kWp figure in Advanced Options for your own location and orientation.
Self-consumption
- Without battery: Typically 30-50% of generation is self-consumed
- With battery: Typically 70-90% of generation is self-consumed
Savings calculation
Daily saving = Stored energy × (Import rate - Export rate). Each kWh you store and then use yourself saves the difference between what you would have paid to import it and what you would have received for exporting it. “Stored energy” here is capped by three limits: the battery's usable capacity, your daily solar surplus, and your actual evening/overnight consumption — a battery cannot save you anything on energy you never use after dark. If your export rate is higher than your import rate, storing solar is no longer worthwhile (exporting pays more), so the tool reports payback as not applicable.
With time-of-use tariffs, extra savings can come from charging the battery from the grid at the off-peak rate and discharging at peak. This only applies to spare cycle capacity not already filled by solar — the same kWh cannot be both solar-charged by day and grid-charged overnight — and the share of cycles where this pays is an installer rule of thumb, not a published figure.
Important notes
This calculator provides estimates based on annual averages. Actual performance varies significantly by season, weather and individual usage patterns. Battery savings are highest in summer when solar generation peaks and lowest in winter.
Frequently Asked Questions
For a typical UK home using 8-10 kWh per day with a 4 kWp solar panel system, a 5-10 kWh battery is usually ideal. If you're out at work all day (low daytime usage), a larger battery captures more surplus solar for evening use. If you work from home and use most solar directly, a smaller battery may suffice. The key is matching the battery to your surplus solar — there's no point buying a 13.5 kWh battery if you only generate 4 kWh of surplus per day.
Popular UK home battery options include the GivEnergy 2.6 kWh and 5.2 kWh (stackable), GivEnergy All-in-One 13.5 kWh, Tesla Powerwall 2 at 13.5 kWh, SolaX Triple Power at 5.8-11.6 kWh, and Fox ESS HV systems from 2.56-10.4 kWh. Most can be stacked or combined to increase capacity. GivEnergy and Tesla are the most popular brands installed by UK solar companies.
Solar battery costs in 2026 are typically around £650-900 per kWh of usable capacity, fully installed, with larger 13 kWh+ systems dropping toward ~£650/kWh and small 2-5 kWh units nearer £900-1,150/kWh. A 5 kWh GivEnergy system costs roughly £3,500-4,500 installed; a 13.5 kWh Tesla Powerwall 2 around £6,000-8,000. Home battery storage in Great Britain currently qualifies for 0% VAT — whether installed with solar or as a standalone/retrofit system — under the energy-saving materials relief, until 31 March 2027 (after which it reverts to 5% VAT).
Self-consumption is the percentage of your solar generation that you use yourself, rather than exporting to the grid. Without a battery, typical self-consumption is 30-50% — the rest is exported for around 4-15p/kWh via the Smart Export Guarantee. With a battery, self-consumption rises to 70-90%, meaning you use your own solar energy instead of buying electricity at the import rate (around 26p/kWh under the Ofgem cap for 1 July-30 September 2026). The bigger the gap between your import and export rates, the more a battery saves; if your export rate ever exceeds your import rate, exporting is actually better than storing.
Yes, if you have a time-of-use tariff like Octopus Agile, Octopus Go, or Intelligent Octopus. These tariffs offer cheap overnight electricity (often 7-10p/kWh) which you can use to charge your battery, then discharge during peak hours when the rate is 24-35p/kWh. This 'tariff arbitrage' can save an additional £200-400 per year on top of your solar savings. Your battery inverter needs to support scheduled charging.
Most home solar batteries have a warranty of 10-12 years or a specified number of charge cycles (typically 6,000-10,000 cycles). At one cycle per day, that's 16-27 years of use. In practice, lithium-ion batteries degrade to about 70-80% of their original capacity after 10 years. Tesla Powerwall 2 is warranted for 10 years at 70% capacity. GivEnergy offers a 12-year warranty.
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Last updated: February 2026
Verified against UK standards · estimates only, confirm with your supplier.