Inverter battery backup calculator
Enter the battery and the watts you need during an outage. The page shows backup time, or the amp-hours to buy for the hours you want, plus an inverter size.
- Hours of backup
- Free, no sign-up
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How to use it
- Enter amp-hours, voltage, battery type and how the batteries are wired.
- Enter the load in watts, or pick appliances that run together.
- Read the hours, or switch the goal to find the amp-hours for a target time.
Usable energy and backup time
Usable watt-hours are amp-hours times volts times the usable depth times inverter efficiency. Lead-acid, tubular and AGM are counted at 50% so the battery is not taken flat. Lithium is counted at 90%. Hours are that energy divided by the load. Series wiring multiplies voltage, parallel multiplies amp-hours, and the energy of the same batteries is the same either way.
The inverter line takes the load at a 0.8 power factor and then steps up to a common VA size with about 25% headroom. Appliance picks use the same watt list as the electricity calculator, and they assume those appliances run together.
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Everything runs in this page and stays on your device. The battery and load stay on this device.
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Questions
How long will a 150 Ah, 12 V battery run 200 W?
At 50% usable depth and 85% inverter efficiency the usable energy is 765 Wh, about 3 hours 50 minutes.
Why is only half of a lead-acid battery counted?
A lead-acid battery lasts longer when it is not drained flat. The page uses 50% of its rated amp-hours. Lithium uses 90%.
What changes between series and parallel?
Series adds the voltages and keeps the amp-hours. Parallel adds the amp-hours and keeps the voltage. Total watt-hours match for the same set of batteries.
What VA inverter should I compare?
Divide the watts by 0.8 for a minimum VA, then take a common size about 25% above that. A 200 W load points at a 400 VA unit.
How do I size the battery for a number of hours?
Choose Battery size for a set of hours, enter the hours and the load. The page returns the amp-hours of each battery and of the whole bank.