Solar Battery Calculator
Size a home battery to store solar surplus and cut evening grid imports.
Please note: Estimates for planning only. Actual solar output depends on your latitude, roof orientation, shading, weather and equipment. Get a site-specific assessment and quotes before committing.
What the Solar Battery Calculator does
A home battery stores midday solar surplus for use after dark. The right size is limited by two things at once: how much surplus you actually generate, and how much you actually use in the evening. Whichever is smaller sets the useful capacity — anything beyond that never gets cycled.
Formula
Daytime use = Daily use × Daytime shareSurplus = max(0, Generation − Daytime use)Useful storage = min(Surplus × Round-trip efficiency, Evening use)Capacity = Useful storage ÷ Depth of discharge ÷ Round-trip efficiencySaving per stored kWh = Import rate − Export rate
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Daily electricity use | kWh | Yes | Accepts more than 0. |
| Daily solar generation | kWh | Yes | Accepts 0 or more. |
| Share of use during daylight | % | Yes | The rest is evening and overnight use a battery could cover. |
| Usable depth of discharge | % | Yes | Lithium batteries typically allow 80–90%. |
| Round-trip efficiency | % | Yes | Energy lost charging and discharging. |
| Days of autonomy wanted | number | Optional | For off-grid or outage backup. Leave at 0 for daily cycling only. Accepts 0 or more. |
| Battery cost per kWh | selected currency | Optional | Accepts 0 or more. |
| Import rate per kWh | selected currency | Yes | Accepts more than 0. |
| Export rate per kWh | selected currency | Yes | Accepts 0 or more. |
| Currency | one of 10 options | Optional | — |
How to use it
- Choose Currency.
- Enter Daily electricity use, Daily solar generation, Share of use during daylight and Usable depth of discharge and 3 more.
- Optionally add Days of autonomy wanted and Battery cost per kWh.
- Select Calculate.
Worked example
30 kWh daily use, 21.6 kWh generated, 40% used in daylight, 90% depth of discharge, 90% round trip.
- Usage
- 30 kWh
- Generation
- 21.6 kWh
- Daytime
- 40
- DoD
- 90
- Round trip
- 90
Daytime use 12 kWh, surplus 9.6 kWh, evening need 18 kWh. Surplus limits storage to 8.64 kWh → a 10.67 kWh battery.
Frequently asked questions
Does a battery pay for itself?
It depends entirely on the gap between import and export rates. Where export pays well, batteries struggle to justify themselves. Where export is near zero, they can pay back in under ten years.
Why is usable capacity lower than the rating?
Batteries reserve part of their capacity to protect cell life. A 13.5 kWh battery at 90% depth of discharge gives about 12.15 kWh usable, less round-trip losses.
Should I size for backup or for daily cycling?
Daily cycling drives the economics; backup drives the size. If outages matter, size for the autonomy you need and accept the extra capacity will not pay back on its own.