How long will a 12 V battery last?
The short answer: divide the battery's usable watt-hours by your load in watts. A 100 Ah, 12 V LiFePO4 battery has roughly 1,000 Wh you can comfortably use, so it runs a 100 W load for about 10 hours. A 100 Ah lead-acid battery gives about half that if you want it to last for years.
"How long will it last?" depends on three numbers: how much energy the battery holds, how much of it you can use, and how fast your devices use it. Here's how to put them together, with worked examples.
The formula
Or, if you know your load in amps at 12 V:
- Ah × volts gives the stored energy in watt-hours. A 12.8 V, 100 Ah LiFePO4 battery stores 1,280 Wh. A 12 V, 100 Ah lead-acid battery stores 1,200 Wh. The Ah to Wh converter does this step.
- Usable fraction is how much of that you plan to use. Lead-acid batteries last much longer if you regularly use only about half their capacity. Lithium (LiFePO4) batteries tolerate much deeper discharge. This guide assumes 80% for lithium, to leave a cautious margin.
- Efficiency covers conversion losses. Devices that run straight from 12 V lose very little. Devices plugged into an inverter lose roughly 10–15%, so this guide uses 85% for them.
How long a 100 Ah battery lasts
Using the assumptions above: 1,280 Wh × 80% = 1,024 usable Wh for LiFePO4, and 1,200 Wh × 50% = 600 usable Wh for lead-acid.
| Load | 100 Ah LiFePO4 (1,024 Wh usable) | 100 Ah lead-acid (600 Wh usable) |
|---|---|---|
| 10 W, 12 V DC (LED lights, a small fan) | about 102 hours | about 60 hours |
| 50 W, 12 V DC | about 20 hours | about 12 hours |
| 100 W, 12 V DC | about 10 hours | about 6 hours |
| 500 W through an inverter | about 1.7 hours | about 1 hour |
| 1,000 W through an inverter | about 50 minutes | about 30 minutes |
Example for the 500 W row: 1,024 Wh × 0.85 ÷ 500 W ≈ 1.74 hours. These are estimates for a steady load. Real runtimes vary with temperature, battery age and how the load changes over time.
The lead-acid figures for heavy loads are optimistic. Lead-acid batteries deliver noticeably less than their rated amp-hours when drained quickly, because ratings usually assume a slow discharge over about 20 hours. Lithium batteries hold their capacity much better under heavy loads.
Worked example: using amps
A 12 V device draws 5 A. On a 100 Ah LiFePO4 battery with 80% usable, that's 80 Ah ÷ 5 A = 16 hours. On a 100 Ah lead-acid battery with 50% usable, it's 50 Ah ÷ 5 A = 10 hours.
If you only know the watts, divide by 12 to get amps: a 60 W device draws about 5 A at 12 V.
Devices that cycle on and off
For fridges, freezers, heaters with thermostats and anything else that switches itself on and off, use the average power over a full day, not the power while it's running. A 12 V compressor fridge might draw 45 W while running but far less on average, depending on the weather, how full it is and how often it's opened. A plug-in or inline power meter shows the real average. Our refrigerator runtime guide explains how to measure it.
What makes a battery run out sooner
- Cold. Batteries deliver less capacity in the cold, and lithium batteries generally can't be charged below freezing. See LiFePO4 vs. NMC for the charging limits.
- Age. Every battery loses capacity over time and with use. An older battery may hold well under its original rating.
- Inverter idle draw. An inverter uses some power just by being switched on, even with nothing plugged in. With small loads this can be a big share of the total. Check the "no-load" or "idle" current on its spec sheet, and switch it off when you're not using it.
- Voltage drop in thin cables. Undersized wiring wastes energy as heat and can make inverters shut down early on "low voltage".
- A device drawing more than you think. Labels show maximum or nominal power. Measuring is the only way to be sure.
What about a car battery?
Car starter batteries are built to deliver a big burst of current for a few seconds to start the engine, not to be drained slowly. Running one down deeply shortens its life, and leaves you unable to start the car. For running devices with the engine off, a separate deep-cycle or lithium battery, or a portable power station, is the better choice. If you do run something small from the car battery, keep it brief.
Frequently asked questions
How long will a 100 Ah battery run a 1,000 W inverter?
At a full 1,000 W load, about 50 minutes from a 100 Ah LiFePO4 battery using 80% of its capacity, and less from lead-acid. Most of the time an inverter runs well below its rating, so work from your actual load, not the inverter's size. The battery must also be able to supply the current: 1,000 W from 12 V is well over 80 A on the battery side.
How many amp-hours do I need?
Turn the formula around: usable Ah needed = load (A) × hours. Then divide by the usable fraction (0.8 for lithium, 0.5 for lead-acid) to get the rated Ah. For example, 5 A for 20 hours = 100 Ah usable, which means about 125 Ah of lithium or 200 Ah of lead-acid. For a full device list, the power station size calculator adds everything up for you.
Is there a calculator for this?
Yes. The battery runtime calculator accepts capacity in Ah or Wh, a load in watts, the output type (DC or inverter) and a reserve, and shows the runtime plus a table for common devices.