The right battery size is not the largest one that fits, and it is not a fixed proportion of your annual electricity use. It is the amount of energy you can realistically shift from cheap or surplus periods into expensive ones, day after day.
Enter your evening demand, solar surplus and charging window below to get a starting capacity. Then check the result against the cost, the warranty and the space you have.
This calculator has several inputs and works best on a tablet or desktop. On a phone you may need to scroll within it.
Nameplate capacity is not usable capacity
A battery advertised as 10kWh will not deliver 10kWh to your home.
Some capacity is held back as a reserve to protect the cells, and more is lost to round-trip conversion losses on the way in and out. The figure that matters is what actually reaches your sockets, which is why two batteries with the same headline number can perform differently.
Ask any installer for the usable capacity in kWh, not just the nameplate rating, and check whether the quoted figure already accounts for conversion losses.
Why bigger is not automatically better
Capacity you never cycle earns nothing. A battery only pays back through the energy it moves, so any portion that sits unused most of the year is dead cost.
Oversizing tends to show up in three ways:
- Summer: the battery fills early in the day and the remaining solar is exported anyway.
- Winter: there is little solar surplus to store, so the battery relies on cheap overnight charging — and only needs enough capacity to cover the evening peak.
- Year round: if your off-peak window is short, charge power may limit how much you can actually put in before the cheap rate ends.
That last point catches people out. A large battery paired with modest charge power may not fill within a short overnight window, so the extra capacity never gets used.
Reading the result
Treat the output as a starting point for conversations with installers, not a specification. The strongest sizing decision comes from your own half-hourly smart-meter data, which shows exactly when you use electricity rather than how much you use in total.
Before you commit to a size
- What Size Home Battery Do I Need? — the full sizing method behind this calculator
- Home Battery kW vs kWh Explained — why charge power can matter more than capacity
- Battery Round-Trip Efficiency — the gap between nameplate and usable energy
- Expandable Home Batteries — starting smaller and adding capacity later
- Smart Meters and Half-Hourly Data — getting the consumption data that sharpens this estimate
- Where Can a Home Battery Be Installed? — physical constraints that may cap your options
See where the stored energy actually goes
Capacity only pays back through the energy it moves. The flow visualiser shows those movements across a day.