Size storage around your evenings, not your bill.
Battery Size Calculator
A home battery earns its place by covering the electricity you use after your solar panels stop generating. Size it too small and you are back on the grid by mid-evening; size it too large and you pay for capacity that never fills. This calculator gives a starting point from your own electricity use.
Enter your annual or monthly use from a recent bill, choose how much of your use falls in the evening, and the calculator matches the result to the Fox ESS EP6 and EP12 batteries we install, including parallel configurations.
Work out your battery size
About 4.8 kWh of usable storage
Closest Fox ESS configuration: 1 x Fox ESS EP6 (5.18 kWh usable).
These are estimates. Your actual figures come from a free home survey.
Get a QuoteHow we calculate this
- Daily use is your annual use divided by 365.
- The share of daily use falling after the panels stop generating is assumed to be 40% (light), 50% (typical) or 60% (heavy evening use).
- Usable capacity is the Fox ESS figure at 90% depth of discharge: 5.18 kWh per EP6 and 10.37 kWh per EP12, with up to four units in parallel.
- Assumptions last checked 19 October 2026.
Why evening use matters more than your bill
Your bill tells you how much electricity you buy, but not when you use it. A battery's job is to shift solar generation from the middle of the day into the evening and overnight. What matters is how much you use once the panels stop generating, which is why the calculator asks about your evening pattern.
A household that is out all day generates into an empty house and uses most of its electricity in the evening, so it benefits from more storage. A household that is home all day uses more of its solar as it is generated, so it needs less.
Fox ESS EP6 and EP12 capacities
The Fox ESS EP6 has 5.76 kWh nominal capacity, with approximately 5.18 kWh usable at 90% depth of discharge. The EP12 has 11.52 kWh nominal, approximately 10.37 kWh usable. Both can be connected in parallel, up to 23.04 kWh nominal for EP6 and 46.08 kWh nominal for EP12.
Both are rated IP65, have a cycle life of at least 6,000 cycles at 25°C, 90% depth of discharge and 70% state of health, a round-trip efficiency of at least 95%, and a 10 year warranty.
When the result needs a closer look
The calculator works from averages. Use changes between summer and winter, and some homes have large loads that move the answer, such as an electric car. Car charging is often better handled by a time-of-use tariff than by a bigger battery, so we look at it separately at the survey.
Battery and solar must also be sized together. A large battery paired with a small array may rarely fill in winter, so the survey checks your roof and your usage before we recommend a configuration.
Why homeowners choose Cozy Home Solutions
Cozy Home Solutions Ltd is an MCS certified installer of solar panels, battery storage and home EV chargers, based at Unit 2, Old Station Yard, Watlington Road, Cowley, Oxford, and working with homeowners across England and Wales. We are registered with NAPIT, HIES and EPVS, and we are a Hometree partner.
Every installation starts with a free home survey and a written quotation. When the work is finished, your system is commissioned and handed over with its MCS certificate, the DNO notification and the manufacturer warranties registered in your name. Battery cover runs to 10 years, and the installation itself is protected by an insurance-backed guarantee.
- Free home survey before any proposal
- DMEGC solar panels, Fox ESS batteries and Fox ESS EV chargers
- MCS certificate, DNO notification and warranties in your name
- Call 0800 688 9666, Monday to Friday, 9:00 to 19:00
Frequently asked questions
What size battery do I need for a three-bedroom house?
Bedroom count is a weak guide. Battery size depends on how much electricity you use after your panels stop generating, so start from your annual use and evening pattern.
Can I add more battery capacity later?
Yes. Fox ESS EP6 and EP12 units can be connected in parallel, up to four units, so capacity can be added later subject to a compatibility check.
How much of a Fox ESS EP12 can I use?
The EP12 has 11.52 kWh nominal capacity, with approximately 10.37 kWh usable at 90% depth of discharge.
Does an electric car mean I need a bigger battery?
Not necessarily. Car charging is often best timed to a lower-rate overnight tariff, so we look at it separately rather than simply adding it to battery size.
Is the calculator result a quote?
No. These are estimates. Your actual figures come from a free home survey.