Solar Battery Size Calculator (Home Backup kWh)
Enter the load and backup time to find the battery capacity you need.
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Solar Battery Size Calculator (Home Backup kWh) by TheCalcTools
How the Solar Battery Size Calculator (Home Backup kWh) works
Energy needed = average load × backup hours. Batteries should not be fully discharged, and the inverter loses some energy, so the nominal capacity must be larger.
Size for your real average load, not the sum of every appliance’s rating, and add margin for cold temperatures and battery ageing.
The formula
Worked example
Using the default values shown in the calculator:
| Average load during backup (W) | 800 |
|---|---|
| Backup time (hours) | 8 |
| Usable depth of discharge | 90 % |
| Inverter and battery efficiency | 90 % |
| Battery bank voltage (V) | 48 |
- 800 W × 8 h = 6.4 kWh delivered.
- ÷ 0.9 DoD ÷ 0.9 efficiency = 7.90 kWh, or 165 Ah at 48 V.
Result:
| Battery capacity (kWh) | 7.90 |
|---|---|
| At your bank voltage (Ah) | 165 |
| Energy delivered (kWh) | 6.40 |
| Days of backup per 10 kWh battery at this load | 0.42 |
Frequently asked questions
How big a battery do I need to run a fridge overnight?
A fridge averages about 50 to 100 W, so 12 hours needs 0.6 to 1.2 kWh delivered, about 0.8 to 1.5 kWh of lithium battery.
Sources and further reading
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This calculator gives estimates for information only. Check important results independently before you rely on them. See our disclaimer.