Backup Battery Size Calculator
Estimate nominal battery capacity required from average essential load, backup duration, usable capacity and inverter efficiency.
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How to choose the inputs
List the essential loads, measure their combined average, and decide how many hours of backup you need. The usable share describes the part of nominal capacity you allow for that event. Account for retained charge only once.
Formula and measurement boundary
Three worked scenarios
These are fixed hypothetical examples, not live tariffs or verified product data. They use EUR and kilometres where applicable and do not change with the form settings. Only the table input changes.
Unchanged inputs
- Average essential load: 500 W
- Usable capacity: 90 %
- Inverter efficiency: 90 %
| Required backup duration | Estimated result |
|---|---|
| 6 hours | 3.7 kWh battery |
| 12 hours | 7.41 kWh battery |
| 24 hours | 14.81 kWh battery |
The loads require 3, 6 and 12 kWh respectively; conversion losses and restricted usable capacity increase the nominal requirement. The additional 15% shown in results is an illustrative margin chosen by this tool, not a design standard or guarantee.
What this result does not establish
- No solar recharge during the outage is assumed.
- A large enough battery may still have insufficient continuous or start-up output power.
- This is not a reliability assessment for medical or other safety-critical loads.
Size the energy budget first, then check output separately
The following are additional hypothetical decision examples. Separately discussed fees, reserve allowances or running costs are not automatically added to the form result.
A constant 300 W essential load for 12 hours requires 3.6 kWh delivered. With 80% usable depth and 90% delivery efficiency, required nominal capacity is 3.6 ÷ (0.8 × 0.9) = 5 kWh. If a separate 10% ageing allowance is wanted, divide by 0.9 again: 5.56 kWh. Do not apply that allowance if the capacity input already represents aged capacity.
A list of appliance nameplate powers can overstate cycling loads and understate starting demand at the same time. Build an hour-by-hour plan: continuous networking, measured refrigerator cycles and a short lighting period. Convert each to Wh, sum the energy, then separately identify which appliances may start together. A kettle used for ten minutes adds 0.333 kWh at 2 kW but still requires 2 kW output while running.
The size result does not choose chemistry, output power, compatible inverter or safe installation. Check available capacity at the intended reserve, temperature and warranty conditions against the specific product. Run a longer-outage scenario and decide which loads can be disconnected before buying capacity for the entire house.
Sources and their scope
- EIA: measuring electricity — Power and energy units only; not equipment performance.
These reference pages were accessed on 21 September 2026. The equations and examples are the site’s own model; these sources do not certify its defaults or results.
Frequently asked questions
Is the 15% margin mandatory?
No. Select an appropriate margin with the system designer using aging, load and reliability requirements.
Does this design the backup connection?
No. Compatible inverter, isolation and protection arrangements require separate professional design.
Read the model and verification limits