Battery storage calculator
A home battery stores daytime solar surplus for the evening and night. Its size follows from your daily use and the share of it you want to cover after the sun goes down.
Your measurements
On your electricity bill, or the annual figure ÷ 12.
Result
- Average daily use
- 11.7 kWh
- Recommended battery capacity
- 5.8 kWh
Exact sizing depends on your hourly consumption profile and the power of the hybrid inverter.
How the calculation works
- Average daily use = monthly use ÷ 30.
- Battery capacity (kWh) = daily use × the share you want to cover from the battery (30–100%).
Worked example
At 350 kWh a month you use on average 11.7 kWh a day. To cover half of it from the battery you need about 5.8 kWh of storage.
What the calculator assumes
- The share approximates evening and night use, when panels don't produce — usually 40–60% of daily use.
- The capacity is usable capacity; LiFePO4 batteries allow discharge to about 90%, so the nominal capacity can be slightly higher.
When to adjust the result
- The battery also needs enough power (kW) for appliances running at the same time, and the inverter must be a hybrid.
- A battery larger than the panels' daily surplus never fills completely — size them together.
Sizing a home battery realistically
A battery pays off by shifting your own solar energy from daytime into the evening and night. Size it on the evening and night consumption you actually have — often 4–8 kWh for a family home — rather than on the total daily use. A battery much larger than the summer surplus sits half-empty most of the year.
Check the usable capacity (not just the nominal one), the maximum charge and discharge power and the warranted cycles or throughput. If you want backup during power cuts, the system needs a backup or island function — many grid-tied systems switch off when the grid fails.
Frequently asked questions
What size battery for a 4 kWp solar system?
Often 5–10 kWh, depending on how much of your evening use you want to cover and how large the daily surplus is.
Can a battery power the house during a blackout?
Only if the system is designed for backup or island operation; many grid-tied systems switch off when the grid fails.
How long do home batteries last?
Lithium iron phosphate batteries are typically warranted for around 10 years or a set number of cycles, with capacity gradually declining over time.
Does a battery power my house in a blackout?
Only if the inverter has a backup or island mode and the circuits are wired for it; otherwise the system shuts down with the grid for safety.