Inverter and UPS battery backup calculator
Useful during power cuts to estimate how long home batteries can run a fridge, lights or a router.
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How to use it
- Enter voltage per battery, capacity per battery, number of batteries, load, inverter efficiency and allowed depth of discharge.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
Hours = voltage × Ah × number of batteries × (depth of discharge ÷ 100) × (efficiency ÷ 100) ÷ load in watts.
Example
Inputs: Voltage per battery = 12 V, Capacity per battery = 100 Ah, Number of batteries = 1, Load = 500 W, Inverter efficiency = 90 %. Result: Backup time: 1.08 h; In minutes: 64.8 min; Energy available to the load: 540 Wh.
Limits and caveats
- Assumes identical batteries; the result is the same whether they are wired in series or parallel.
- Real capacity drops at high discharge rates (Peukert effect) and with age, so expect less with heavy loads.
- The default 50% depth of discharge suits lead-acid batteries; lithium batteries usually allow 80 to 90%.
Questions about this tool
How long does a 12 V 100 Ah battery run 500 W?
About 1 hour 5 minutes at 90% efficiency and 50% discharge. Full discharge doubles that but shortens battery life.
Why not drain the battery to zero?
Repeated deep discharge greatly shortens lead-acid battery life, so staying above about 50% is advised.
How do I find the load in watts?
Add up the rated power on the label of every device running at the same time. Motor loads draw more at start-up.