How this calculator works
A battery's energy in watt-hours is its capacity in amp-hours times its voltage. Only part of it is usable, because discharging a lead-acid battery deeply shortens its life, so multiply by the usable share. The runtime is the usable energy times the inverter efficiency, divided by the load in watts. Batteries in parallel add their capacities.
Worked example: a 12 V 100 Ah battery stores 1,200 Wh (1.2 kWh). At 50% usable that is 600 Wh, which runs a 300 W load for 1.8 hours through a 90% inverter, or 2.0 hours on a DC load. A 90% usable lithium battery of the same size runs it 3.2 hours, two batteries in parallel 3.6 hours, and a 24 V battery of 100 Ah stores 2,400 Wh. A 2,000 Wh battery at 12 V is 167 Ah.
Energy in common batteries
Watt-hours and hours of runtime for a 300 W load (50% usable, 90% inverter).
| Battery | Watt-hours | Usable (50%) | Hours at 300 W |
|---|---|---|---|
| 12 V 50 Ah | 600 | 300 | 0.9 |
| 12 V 100 Ah | 1,200 | 600 | 1.8 |
| 12 V 200 Ah | 2,400 | 1,200 | 3.6 |
| 12 V 300 Ah | 3,600 | 1,800 | 5.4 |
| 24 V 100 Ah | 2,400 | 1,200 | 3.6 |
| 48 V 100 Ah | 4,800 | 2,400 | 7.2 |
| 48 V 200 Ah | 9,600 | 4,800 | 14.4 |
Tips before you buy
- A "100 Ah" battery is 1,200 watt-hours at 12 volts, 2,400 at 24 volts and 4,800 at 48 volts: the voltage matters.
- Compare batteries in watt-hours (or kWh), not amp-hours, if they are different voltages.
- A lead-acid battery gives about half its rated capacity before it is damaged. A LiFePO4 battery gives 80 to 90% and many more cycles.
- Heavy loads shorten the runtime more than the sum suggests, and cold weather cuts the capacity.
- For a portable power station use the manufacturer's watt-hours and our power station runtime calculator.
Tools & materials for this project
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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.
How long will a 100Ah battery run a 300 watt load?
About 1.8 hours with a lead-acid battery limited to 50% usable and a 90% inverter, or 3.2 hours with a lithium battery using 90%.
How many kWh is a 12 V 100 Ah battery?
1.20 kWh (1,200 Wh).
How many amp-hours is 2,000 watt-hours?
167 Ah at 12 V, 83 Ah at 24 V.
How much of a battery can I use?
About half for lead-acid and AGM batteries to get a long life, and 80 to 90% for lithium iron phosphate. Using more shortens the life.
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