Solar Battery Storage Size Calculator
Calculate the right solar battery storage size for your home in kWh.
Enter daily energy use, backup days needed, and depth of discharge to find capacity.
A solar battery stores surplus energy generated during the day for use at night or during cloudy periods. Sizing it correctly avoids paying for unused capacity while ensuring you have enough storage to cover your evening demand.
Battery sizing formula: Usable battery size (kWh) = evening/night load (kWh) / depth of discharge (DoD)
Depth of discharge:
- Lithium iron phosphate (LFP): 90–95% DoD, and the one to pick
- NMC lithium: 80–90% DoD
- Lead-acid: 50% DoD (avoid for solar use)
Worked example: Evening load (18:00–08:00): 8 kWh LFP battery at 92% DoD: Battery size needed = 8 / 0.92 = 8.7 kWh → buy a 10 kWh battery
Popular home battery sizes:
- 5 kWh: Small flat or supplemental storage
- 10 kWh: Average house, covers most evenings
- 15 kWh: Large house or EV top-up overnight
- 20+ kWh: Off-grid or EV-heavy households
Days of autonomy is a different question, and it needs a different number.
One day of storage means covering tonight, from sundown to the moment the panels wake up. Your evening load is the right input for that, because tomorrow morning the sun handles the daytime again.
Two or three days means the sun does not come back properly. On a genuinely overcast day the panels might give you a fifth of nameplate, so the battery is carrying the daytime load as well: the fridge, the heating pump, whatever runs while you are awake. That is your total daily consumption, not the evening slice of it, and for most households the total is roughly double the evening figure.
Size a three-day system off an 8 kWh evening load and you get 26 kWh. Size it off the 16 kWh daily total it actually implies and you get 52 kWh. Getting this wrong is not a rounding error, it is half a battery.
Cost: battery prices move fast and vary hugely by country, so this page asks you for your own installed cost per kWh rather than printing a figure that will be stale in a year. Take it off a quote. If you have not got one yet, run the calculation without it: the kWh figure is the part you take shopping.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
SuperGlobalCalculator is independently built and maintained. See how we build and verify our calculators.