Estimated backup time
4.8 hours
Uses an 80% combined usable-energy allowanceHow this calculator helps
Estimate how long a battery bank can support a known load during an outage. The result applies a conservative usable-capacity and conversion-loss allowance so it is more useful than simply dividing watt-hours by watts.
How to use it
- 1
Add the running watts of every appliance that will operate during backup.
- 2
Enter the battery bank's rated amp-hours and total voltage.
- 3
Use the estimated time conservatively and avoid discharging batteries beyond manufacturer guidance.
Formula and methodology
Nominal energy = amp-hours ร voltage. Estimated runtime = nominal energy ร usable-capacity factor ร inverter efficiency รท connected load.
Worked calculation example
A 150 Ah, 12 V battery stores 1,800 nominal watt-hours. At 80% combined usable energy and conversion efficiency with a 300 W load, estimated runtime is about 4.8 hours.
What can affect the result?
Battery chemistry
Lead-acid and lithium batteries have different safe depth-of-discharge limits.
Battery health
Age, heat and repeated deep discharge reduce available capacity.
Conversion losses
Inverters and wiring consume energy before it reaches appliances.
Limits of this estimate
Runtime is an energy estimate, not a battery guarantee; it omits chemistry-specific discharge curves, surge loads and inverter cut-off behaviour.
Frequently asked questions
Is the battery backup calculator free?๏ผ
Yes. SolvePilot tools are free, work in your browser, and require no account.
Does SolvePilot store my values?๏ผ
No. Calculations run locally in your browser and the values you enter are not sent to a server.
Should I treat the result as an exact bill or quotation?๏ผ
No. Results are informed estimates. Final charges can vary because of taxes, tariffs, lender terms, usage patterns, or provider rules.
How can I improve the accuracy?๏ผ
Use recent values from your own bill, statement or institution, review every assumption, and confirm important results with the relevant official source.
Why is actual backup sometimes shorter?๏ผ
Battery age, temperature, discharge rate, wiring losses and inverter cut-off voltage can all reduce usable runtime.