How long will a 100Ah battery run a fridge?

A 100Ah lithium battery has about 90Ah usable, and a compressor fridge draws roughly 5A while its compressor runs. Because the compressor cycles rather than running continuously, the fridge's average draw is well below that, so the battery usually covers a full day of fridge use. The same 100Ah in lead-acid is only good for about half its capacity, so it lasts noticeably less.

Should I discharge a lithium battery to 0%?

No. A lithium battery can be worked to about 90% depth of discharge, but taking it to 0% is not the same thing and gives you nothing in return except a battery that is empty when you need it. Treat what is left as a floor and recharge before it gets there.

Does cold weather reduce battery capacity?

Yes. A cold battery delivers less of its rated capacity, so a load that would run for hours in summer runs for fewer in winter. The effect is temporary rather than permanent for a battery that is only discharged cold; it is charging a lithium battery below freezing that causes lasting damage.

Why does my battery run down faster than the calculation says?

Usually because the calculation left something out. Inverter losses add to every AC load, appliances that cycle still draw their full current while running, and capacity falls with cold and with age. A load that is quoted in watts at the appliance also draws its conversion losses from the battery, so the real current is always a little higher than watts divided by 12.

How long will a 12V battery run my appliances?

6 min read

Runtime comes from three numbers: the battery's usable capacity in amp-hours, the load in amps, and how deeply you are willing to discharge. Convert the load to amps by dividing its wattage by 12, then divide the usable capacity by that figure. A 100Ah lithium battery can be taken to about 90% depth of discharge without harm, so roughly 90Ah is usable; running a 5A load, about 60W, gives around 18 hours. The same 100Ah label on a lead-acid battery is only safe to about half, so the same load lasts around 10 hours — which is the difference that catches people out.

Convert the load to amps first

Runtime is an arithmetic problem before it is anything else. Battery capacity is quoted in amp-hours, so the load has to be expressed in amps as well: divide the appliance's wattage by the battery's nominal voltage, which is 12. That gives the current the load draws, and from there the usable capacity divided by the current gives the hours.

Step	Figure
Load	60W
Current at 12V (60 ÷ 12)	5A
Usable capacity of a 100Ah lithium battery	about 90Ah
Runtime (90Ah ÷ 5A)	about 18 hours

Usable capacity is not the label on the case

The number printed on a battery is its total capacity, not the amount you can safely take out of it. A lithium battery can be worked to about 90% depth of discharge without harm, so a 100Ah lithium battery gives roughly 90Ah of usable energy. A lead-acid battery is only safe to about half its rated capacity, so the same 100Ah label gives roughly 50Ah of usable energy. That single difference is why an identical amp-hour rating can produce very different runtimes.

What makes the real number lower

The real figure falls faster than the label suggests: the heavier the load, the less of the nominal capacity is available.

The calculation gives an upper bound. An inverter wastes some of the energy on the way from DC to AC, so a load quoted at 60W draws more than 60W from the battery. Cold reduces the capacity a battery can deliver, and capacity also falls slowly as the battery ages. A fridge makes the arithmetic look worse in a different way, because its compressor cycles on and off rather than running continuously, so its average draw over an hour is lower than its running draw — but any hour in which it runs steadily draws the full figure.

A worked example

Take a 100Ah lithium battery with about 90Ah usable. A compressor fridge that draws about 60W — 5A while the compressor runs — does not run continuously, so over a full day it uses a fraction of what its running current would suggest. Add a laptop charged for a few hours, and the combined load still sits below a steady 5A average, which is why the battery lasts roughly a day rather than the 18 hours of the steady-load figure. Run the same pair on a 100Ah lead-acid battery at about half usable capacity and the same day's load brings it much closer to empty.

Frequently asked questions

How long will a 100Ah battery run a fridge?: A 100Ah lithium battery has about 90Ah usable, and a compressor fridge draws roughly 5A while its compressor runs. Because the compressor cycles rather than running continuously, the fridge's average draw is well below that, so the battery usually covers a full day of fridge use. The same 100Ah in lead-acid is only good for about half its capacity, so it lasts noticeably less.
Should I discharge a lithium battery to 0%?: No. A lithium battery can be worked to about 90% depth of discharge, but taking it to 0% is not the same thing and gives you nothing in return except a battery that is empty when you need it. Treat what is left as a floor and recharge before it gets there.
Does cold weather reduce battery capacity?: Yes. A cold battery delivers less of its rated capacity, so a load that would run for hours in summer runs for fewer in winter. The effect is temporary rather than permanent for a battery that is only discharged cold; it is charging a lithium battery below freezing that causes lasting damage.
Why does my battery run down faster than the calculation says?: Usually because the calculation left something out. Inverter losses add to every AC load, appliances that cycle still draw their full current while running, and capacity falls with cold and with age. A load that is quoted in watts at the appliance also draws its conversion losses from the battery, so the real current is always a little higher than watts divided by 12.