Enter the battery capacity in amp-hours, the voltage, the load in watts and the usable share of capacity as a percent. Capacity times voltage gives watt-hours. Lead-acid batteries should only be discharged about 50% to preserve life, and lithium iron phosphate batteries allow 80% to 90%.
The load is the draw of the equipment you will run, and inverters add a loss of 5% to 15%.
Battery Runtime: worked example
| Battery capacity (amp-hours) | 100 |
|---|---|
| Battery voltage | 12 |
| Load (watts) | 200 |
| Usable share of capacity | 80.0% |
| Runtime | 4.8 hours |
| Usable energy (watt-hours) | 960 |
Battery Runtime: changing Battery capacity (amp-hours)
| Battery capacity (amp-hours) | Runtime | Usable energy (watt-hours) |
|---|---|---|
| 80 | 3.8 hours | 768 |
| 90 | 4.3 hours | 864 |
| 100 (example) | 4.8 hours | 960 |
| 110 | 5.3 hours | 1,056 |
| 120 | 5.8 hours | 1,152 |
Battery Runtime: compare input ranges
| Input | Tested from | To | Runtime: low input | Runtime: high input | Swing |
|---|---|---|---|---|---|
| Load (watts) | 180 | 220 | 5.3 hours | 4.4 hours | 1.0 hours (20%) |
| Battery capacity (amp-hours) | 90 | 110 | 4.3 hours | 5.3 hours | 1.0 hours (20%) |
| Battery voltage | 11 | 13 | 4.4 hours | 5.2 hours | 0.8 hours (17%) |
| Usable share of capacity | 78.0% | 82.0% | 4.7 hours | 4.9 hours | 0.2 hours (5.0%) |
Battery Runtime: individual input comparisons
| Input | Value | Runtime | Usable energy (watt-hours) |
|---|---|---|---|
| Battery capacity (amp-hours) | 80 | 3.8 hours | 768 |
| 90 | 4.3 hours | 864 | |
| 100 | 4.8 hours | 960 | |
| 110 | 5.3 hours | 1,056 | |
| 120 | 5.8 hours | 1,152 | |
| Battery voltage | 10 | 4.0 hours | 800 |
| 11 | 4.4 hours | 880 | |
| 12 | 4.8 hours | 960 | |
| 13 | 5.2 hours | 1,040 | |
| 14 | 5.6 hours | 1,120 | |
| Load (watts) | 160 | 6.0 hours | 960 |
| 180 | 5.3 hours | 960 | |
| 200 | 4.8 hours | 960 | |
| 220 | 4.4 hours | 960 | |
| 240 | 4.0 hours | 960 | |
| Usable share of capacity | 76.0% | 4.6 hours | 912 |
| 78.0% | 4.7 hours | 936 | |
| 80.0% | 4.8 hours | 960 | |
| 82.0% | 4.9 hours | 984 | |
| 84.0% | 5.0 hours | 1,008 |
Battery Runtime: combined input changes
| Battery capacity (amp-hours) / Battery voltage | 10 | 12 | 14 |
|---|---|---|---|
| 80 | 3.2 hours | 3.8 hours | 4.5 hours |
| 90 | 3.6 hours | 4.3 hours | 5.0 hours |
| 100 | 4.0 hours | 4.8 hours | 5.6 hours |
| 110 | 4.4 hours | 5.3 hours | 6.2 hours |
| 120 | 4.8 hours | 5.8 hours | 6.7 hours |
Battery Runtime: which assumptions move the result?
Battery Runtime: values and definitions
| Quantity and definition | Value used |
|---|---|
| Battery capacity (amp-hours) | 100 |
| Battery voltage | 12 |
| Load (watts) — The total power the connected devices draw. | 200 |
| Usable share of capacity — Lead-acid batteries are usually kept above 50 percent charge; lithium can use 80 to 100 percent. | 80.0% |
| Runtime | 4.8 hours |
| Usable energy (watt-hours) | 960 |
Battery Runtime: half and double Load (watts)
| Load (watts) | Runtime |
|---|---|
| 100 | 9.6 hours |
| 200 | 4.8 hours |
| 400 | 2.4 hours |
High loads reduce the effective capacity of lead-acid batteries, an effect called the Peukert effect. Cold weather cuts capacity. Batteries age and lose capacity over the years.
Inverter losses and the load's real draw vary. Batteries in series raise voltage, while batteries in parallel raise amp-hours.
Battery Runtime: formula
Common error: Multiplying amp-hours by hours without the voltage. Amp-hours alone do not compare across 12 and 24 volt systems, and watt-hours do.
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A battery stores energy, so runtime is stored energy divided by how fast the load drains it. A 12 volt, 100 amp-hour battery holds 1,200 watt-hours, but only 960 are usable at 80 percent. A 200 watt load then runs it for 4.8 hours.
Double the load and the runtime halves, which is why a small draw can outlast a large one by a wide margin.
Set the usable share to match the battery's chemistry. Draining a lead-acid battery deeply shortens its life even if it still delivers power.
Choose a generator on the generator size calculator, offset the load with the solar panel output calculator and estimate running costs on the electricity cost calculator. The watts to amps and Ohm's law pages relate current and power, and the EV charging and LED savings calculators cover related energy questions.
How much of a battery's capacity can I use?
Around 50% for lead-acid batteries and 80% to 90% for lithium iron phosphate to keep their life span.
How long will a 100Ah battery run a 200 watt load?
About 4.8 hours for a 12 volt battery with 80% usable capacity, since 960 watt-hours divided by 200 watts is 4.8.
How do I convert amp-hours to watt-hours?
Multiply the amp-hours by the battery voltage, so a 100 amp-hour 12 volt battery holds 1,200 watt-hours.