Battery Backup Calculator
Size home battery storage for a target backup runtime on critical loads.
About Battery Backup Calculator
Nameplate capacity always exceeds deliverable energy, because neither full discharge nor lossless conversion is available. Combining the two factors typically means about 80 per cent of nameplate reaches the load. Sizing on nameplate capacity alone overstates runtime by roughly a fifth, which is the difference between lasting the night and not.
Formula
Battery Backup
Nominal capacity = load × runtime ÷ (depth of discharge × round trip efficiency)
- depth of discharge
- Share of nameplate capacity safely usable
- round trip efficiency
- Energy out divided by energy in, after conversion losses
Worked example
With the default values below, Battery Backup Calculator returns Nominal Capacity Required of 13.333. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Critical loadminimum 0 W
- 900 W
- Target runtimeminimum 0.1 h
- 12 h
- Usable depth of dischargeaccepts 10–100 %
- 90 %
- Round trip efficiencyaccepts 30–100 %
- 90 %
- Capacity per battery moduleminimum 0.1 kWh
- 5 kWh
Results
- Nominal Capacity Required
- 13.333 kWh
- Usable Energy Needed
- 10.800 kWh
- Battery Modules Required
- 3
- Installed Capacity
- 15.00 kWh
- Runtime Achieved
- 13.50 h
Step by step
- 1Values used
- 2Battery Backup
- 3Nominal Capacity Required= 13.333 kWh
- 4Usable Energy Needed= 10.800 kWh
- 5Battery Modules Required= 3
- 6Installed Capacity= 15.00 kWh
- 7Runtime Achieved= 13.50 h
Assumptions and limits
- The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax or compounding frequency into account.
- Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Battery Backup calculated?
Nominal capacity = load × runtime ÷ (depth of discharge × round trip efficiency) Nameplate capacity always exceeds deliverable energy, because neither full discharge nor lossless conversion is available. Combining the two factors typically means about 80 per cent of nameplate reaches the load.
Why does Battery Backup matter?
Sizing on nameplate capacity alone overstates runtime by roughly a fifth, which is the difference between lasting the night and not.
What values do I need to enter?
This calculator takes 5 inputs: Critical load, Target runtime, Usable depth of discharge, Round trip efficiency, Capacity per battery module. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.