Battery Backup Size Calculator
Estimate battery capacity for home backup loads and outage duration.
This page is an editable estimator with realistic defaults so you can model your own situation in minutes.
Battery kWh = critical load watts x backup hours / 1,000 / usable depth of discharge.
Treat the result as a planning estimate and verify important numbers with the relevant official source before acting.
| Reserve margin | 20% | |
|---|---|---|
| Usable depth | 90% | |
| Inverter efficiency | 92% |
Assumptions
- Backup sizing depends on actual load measurements, motor starting surge, wiring, transfer equipment, and local electrical code.
- Do not connect generators or batteries to home wiring without approved transfer equipment and qualified installation.
- Critical loads should be verified from appliance nameplates or measured power use.
How this calculator works
Formula used
Battery kWh = critical load watts x backup hours / 1,000 / usable depth of discharge.
Example calculation
Plug in your own numbers and the result updates instantly. Battery kWh = critical load watts x backup hours / 1,000 / usable depth of discharge.
Battery Backup Size Calculator FAQ
Is this battery backup size calculator exact?
No. It is an estimate for planning only. Actual bills, savings, and sizing depend on utility tariffs, weather, usage behavior, equipment efficiency, installation quality, local code, and product specifications.
What electricity rate should I enter?
Use your all-in average rate if possible: supply, delivery, taxes, and riders divided by total kWh. Time-of-use plans, demand charges, fixed fees, and tiered rates can change the result.
Should I verify before buying equipment?
Yes. Confirm solar, battery, generator, HVAC, water heater, insulation, window, and electrical decisions with qualified installers, electricians, utility programs, and local permitting requirements.