12V Battery Runtime Calculator

Chemistry moves this answer more than anything else on the page. A flooded, AGM or gel battery hands back less of its label capacity the harder you pull, so a 100Ah battery is only a 100Ah battery at the 20-hour rate its label was measured at. A LiFePO4 battery holds its rating up to a 1C draw and sits at 12.8V nominal rather than 12V. This chart runs a 100Ah 12V battery at each chemistry's maker-printed depth of discharge and 90% system efficiency:

Battery size still scales the answer, but not the way most charts imply. LiFePO4 scales straight: at 100W, 50Ah gives 4.6 hours, 100Ah gives 9.2 and 200Ah gives 18.4. AGM does better than straight, because a bigger battery is also a gentler draw per amp-hour: at the same 100W, 50Ah gives 2.3 hours, 100Ah gives 5 and 200Ah gives 10.8, so doubling capacity multiplies runtime by about 2.2 rather than 2. If your bank is AGM rather than lithium, our AGM runtime calculator starts from AGM defaults instead.

The cells at the far corners of the chart sit outside the rows the makers print: the 25W and 50W AGM and gel figures are lighter than the 20-hour rate the printed tables begin at, and the 500W flooded figure is heavier than the fastest row its maker prints. Below the rows the calculator holds the slowest printed row's capacity, above them it extends the slope of the last two printed rows, and it says so on screen when your own numbers land there. Running loads on 12V DC without an inverter removes the conversion stage entirely, which leaves only wiring loss and stretches every figure above.

Each row is one named product's rating as its maker prints it, and your own appliance's nameplate is the figure to use. Where a maker prints amps, the watt figure is that current times 12 V, our arithmetic; the runtime column divides 921.6 Wh by each watt figure and rounds down. The LED kit, the two Apple adapters and the TV are mains products, so each runs through the inverter named in its row, with the inverter zero load power added; the other four rows are 12 V products that run direct from the battery.

Runtimes above are for a 100Ah 12V LiFePO4 battery at 80% DoD and 90% system efficiency, which delivers 922Wh. The same 100Ah label in AGM at 50% DoD delivers a little over half as long at these loads, and every figure falls further in the cold.

Scenario: you are car camping for two nights with a 100Ah LiFePO4 battery. Two days of loads look like this:

• LED lights: 10W for 5 hours a night = 100Wh over two nights
• Phone charging (2 phones): 20W for 3 hours a night = 120Wh
• 12V fridge: 40W average, running 48 hours = 1,920Wh
• Portable speaker: 5W for 6 hours a night = 60Wh

Total weekend energy: 100 + 120 + 1,920 + 60 = 2,200Wh. Your 100Ah 12V LiFePO4 battery stores 1,280Wh, 1,024Wh of that at 80% DoD, and 922Wh after the 90% system efficiency the calculator applies. That leaves you 1,278Wh short, and the fridge is almost all of it.

Check any remedy against that number rather than against instinct. A second identical battery at 80% DoD brings you to 1,843Wh, which is still 357Wh short. Battle Born prints 100% depth of discharge for its LiFePO4, and two batteries taken to full depth deliver 2,304Wh, which covers the weekend with about 100Wh spare. The cheaper fix is the fridge: at the 40% duty cycle our worked example below uses, a 50W compressor averages 20W, the fridge drops from 1,920Wh to 960Wh, and two batteries at 80% DoD cover the weekend with 600Wh to spare. A portable solar panel helps as well, though how much it makes in a day depends on the panel, the season and the sky.