What the animation shows
The animation and the step-by-step video above use your numbers. Here is the same walkthrough written out for the default example.
What you enter
- Energy needed per day of backup: 10 kWh.
- Days of backup: 1.
- Usable depth of discharge: 90 %.
- Inverter / round-trip efficiency: 90 %.
What happens, step by step
- Each green band is one day: 10 kWh plus 10% inverter loss.
- Halfway through the outage, half the usable energy is gone.
- It stops at the 10% reserve, so you need 12.3 kWh in total.
The answer: battery capacity (nominal) is 12.3 kWh.
How it’s calculated
- Usable = daily kWh × days ÷ efficiency
- Nominal = usable ÷ depth of discharge
Worked example
10 kWh per day for 1 day, 90% depth of discharge, 90% efficiency.
Inputs
- Energy needed per day of backup
- 10 kWh
- Days of backup
- 1
- Usable depth of discharge
- 90 %
- Inverter / round-trip efficiency
- 90 %
Results
- Battery capacity (nominal)
- 12.3 kWh
- Usable energy required
- 11.1 kWh
Assumptions and limits
- No solar recharge during the backup period (conservative).
- Battery capacity loss with age and cold temperatures is not included.
Verify the result against the applicable local code, adopted code edition, manufacturer instructions and project requirements. See our methodology.
Questions
How big a battery do I need to run my house?
List the essential loads, add up their daily kWh, and size for the days of backup you want. Whole-home backup for an average US home typically needs 25–30 kWh per day.
What is depth of discharge?
The share of rated capacity you can use without harming the battery. Many lithium systems allow about 90%.