How Long Will a 12V Battery Last? Ah to Wh and Runtime Chart
Part of the project planner: Planning backup power, batteries and solar.
A battery's energy in watt-hours is its amp-hours times its voltage: a 12 V 100 Ah battery stores 1,200 Wh (1.2 kWh). If you can use half of it, that is 600 Wh, which runs a 300 W load for about 1.8 hours through a 90% efficient inverter. The battery capacity calculator does your own battery and load.
Runtime of common batteries at 300 watts
Lead-acid (50% usable) and lithium iron phosphate (90% usable), 90% inverter efficiency:
| Battery | Watt-hours | Hours (lead-acid) | Hours (lithium) |
|---|---|---|---|
| 12 V 50 Ah | 600 | 0.9 | 1.6 |
| 12 V 100 Ah | 1,200 | 1.8 | 3.2 |
| 12 V 200 Ah | 2,400 | 3.6 | 6.5 |
| 12 V 300 Ah | 3,600 | 5.4 | 9.7 |
| 24 V 100 Ah | 2,400 | 3.6 | 6.5 |
| 48 V 100 Ah | 4,800 | 7.2 | 13.0 |
| 48 V 200 Ah | 9,600 | 14.4 | 25.9 |
Runtime for common loads
A 12 V 100 Ah battery through a 90% inverter:
| Load | Watts | Lead-acid (50%) | Lithium (90%) |
|---|---|---|---|
| LED lights and phone charging | 50 W | 10.8 h | 19.4 h |
| Laptop and router | 100 W | 5.4 h | 9.7 h |
| Small fridge (average draw) | 150 W | 3.6 h | 6.5 h |
| TV and gaming | 250 W | 2.2 h | 3.9 h |
| CPAP and fan | 60 W | 9.0 h | 16.2 h |
| Coffee maker | 1,000 W | 0.5 h | 1.0 h |
How to work it out
- Energy: Wh = Ah × V.
- Usable energy: Wh × the usable share (0.5 for lead-acid, 0.8 to 0.9 for lithium).
- Runtime: usable Wh × inverter efficiency ÷ load in watts.
- Parallel and series: batteries in parallel add capacity at the same voltage; in series they add voltage at the same capacity.
Why real runtimes are shorter
- Heavy loads take more out of a battery than the rating suggests (the Peukert effect), especially lead-acid.
- Cold weather cuts the capacity, sometimes by a quarter or more.
- Old batteries hold less than new ones.
- Inverters use power even with nothing connected, typically 10 to 50 watts.
See also the power station size guide and the power station runtime calculator for portable units.
Frequently asked questions
How do you convert amp-hours to watt-hours?
Multiply by the voltage. 100 Ah at 12 V is 1,200 Wh (1.2 kWh).
How long will a 100Ah battery run a 300 watt load?
About 1.8 hours with a lead-acid battery (50% usable) and a 90% inverter, or 3.2 hours with a lithium battery using 90%.
How many amp-hours do I need?
Divide the daily watt-hours you use by the battery voltage and by the usable share. A 1,200 Wh day on a 12 V lead-acid bank (50%) needs 200 Ah.
Why is a 12V 100Ah battery not 100 amps for an hour at 120 volts?
Because it is 12 volts: 100 amps at 12 volts is only 1,200 watts for an hour. An inverter turns it into 120 volts at a lower current of about 10 amps for the same power, less losses.
How much of a battery can I use?
About half for lead-acid and AGM batteries if you want a long life, and 80 to 90% for lithium iron phosphate.
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