Battery Runtime Calculator
Enter the battery's capacity in Wh, the usable share and the inverter efficiency, then list your devices with watts and hours of use. See how long the battery lasts, how long it runs everything at once, and which device uses the most energy. Planning estimate.
Frequently asked questions
What are usable capacity and inverter efficiency?
A battery should not be run to zero, and many stop at 10 to 20 % to protect themselves, so only part of the printed Wh can be used. An inverter that turns the battery's DC into mains AC wastes some energy as heat, typically 5 to 15 %. Both shrink the energy your devices really get. Devices powered directly from the battery's 12 V or USB outputs skip the inverter: set the efficiency to 100 for those.
How do I find a device's watts?
Look at the label or charger for watts, or multiply volts by amps. For a fridge, freezer or heat pump that cycles on and off, use its average draw (often one third to one half of the label figure) or lower the hours to the hours the compressor really runs.
Why are there two runtimes?
One is how long the battery would last if every listed device ran at the same time, the worst case. The other follows your daily pattern: devices used for a few hours a day last much longer, so the battery covers more of a day. It is shown as hours of the pattern, and in days when it lasts past 24 hours.
Does cold weather or battery age change it?
Yes, and this estimate ignores both. Lithium batteries give noticeably less in the cold, and an older battery holds less than its label. Keep some margin: planning for 20 % less than the result is sensible.
How do I turn amp-hours into watt-hours?
Multiply by the battery's voltage. A 12 V 100 Ah battery holds 1,200 Wh; a 24 V 50 Ah battery also holds 1,200 Wh. Lead-acid batteries should only be used down to about 50 %, so set usable to 50 for them.