Answer first
What this calculator tells you
Estimate how many hours a battery can run a load from its amp-hours, voltage and usable share. Size a battery bank for an RV, a workshop or a backup power supply before you buy it. Formula: Runtime (hours) = capacity (Ah) × voltage × usable share ÷ load (watts). At the worked-example inputs, the runtime is 4.8 hours. Holding every other input steady, moving battery capacity (amp-hours) from 80 to 120 moves the result from 3.8 hours to 5.8 hours.
Transparent method
The formula
Size a battery bank for an RV, a workshop or a backup power supply before you buy it.
Worked example
Example inputs
How to interpret the result
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.
At the worked-example inputs the runtime is 4.8 hours. It rises with battery capacity (amp-hours), battery voltage and usable share of capacity and falls as load (watts) increases.
Real energy use depends on how the appliance or vehicle is actually run: duty cycle, weather, driving style, battery age and your utility's rate structure, including tiers and time-of-use pricing. Treat the result as a planning estimate and check it against your own bill or trip log.
Before you rely on it
What to check
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.
The common error
Where people go wrong with battery runtime calculator
Multiplying amp-hours by hours without the voltage. Amp-hours alone do not compare across 12 and 24 volt systems, and watt-hours do.
Sensitivity evidence
How battery capacity (amp-hours) changes the runtime
Holding every other input at the worked-example value, moving battery capacity (amp-hours) from 80 to 120 moves the runtime from 3.8 hours to 5.8 hours: a spread of 1.9 hours, or 40% of the worked-example result.
| Battery capacity (amp-hours) | Runtime | Usable energy (watt-hours) |
|---|---|---|
| 80 | 3.8 hours | 768 |
| 90 | 4.3 hours | 864 |
| 100worked example | 4.8 hours | 960 |
| 110 | 5.3 hours | 1,056 |
| 120 | 5.8 hours | 1,152 |
Every input, tested
Which input moves the runtime most
Of the 4 inputs, load (watts) moves the runtime most (1.0 hours across the range tested) and usable share of capacity moves it least (0.2 hours).
| Input | Tested from | To | Runtime at each end | Swing |
|---|---|---|---|---|
| Load (watts) | 180 | 220 | 5.3 hours to 4.4 hours | 1.0 hours (20%) |
| Battery capacity (amp-hours) | 90 | 110 | 4.3 hours to 5.3 hours | 1.0 hours (20%) |
| Battery voltage | 11 | 13 | 4.4 hours to 5.2 hours | 0.8 hours (17%) |
| Usable share of capacity | 78.0% | 82.0% | 4.7 hours to 4.9 hours | 0.2 hours (5.0%) |
Two variables at once
Runtime by battery capacity (amp-hours) and battery voltage
Across the grid the runtime runs from 3.2 hours to 6.7 hours. Moving battery capacity (amp-hours) from 80 to 120 shifts it by 1.9 hours at the middle column, and moving battery voltage from 10 to 14 shifts it by 1.6 hours at the middle row, so battery capacity (amp-hours) is the bigger lever here.
| 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 |
The highlighted cell is the worked example: 4.8 hours.
Step by step
The worked example, input by input
| Input | Value used | What it means |
|---|---|---|
| Battery capacity (amp-hours) | 100 | Enter the battery capacity (amp-hours) used in this calculation. |
| Battery voltage | 12 | Enter the battery voltage used in this calculation. |
| Load (watts) | 200 | The total power the connected devices draw. |
| Usable share of capacity | 80.0% | Lead-acid batteries are usually kept above 50 percent charge; lithium can use 80 to 100 percent. |
| Runtime | 4.8 hours | |
| Usable energy (watt-hours) | 960 | |
Inputs, definitions and assumptions
Battery capacity (amp-hours)
Enter the battery capacity (amp-hours) used in this calculation. The prefilled worked-example value is 100.
Battery voltage
Enter the battery voltage used in this calculation. The prefilled worked-example value is 12.
Load (watts)
The total power the connected devices draw. The prefilled worked-example value is 200.
Usable share of capacity
Lead-acid batteries are usually kept above 50 percent charge; lithium can use 80 to 100 percent. The prefilled worked-example value is 80.0%.
How to use this calculator
- 1Verify the inputs. Gather battery capacity (amp-hours), battery voltage, load (watts) and usable share of capacity from your own documents; the prefilled values are examples.
- 2Save a baseline. The worked example puts the runtime at 4.8 hours. Store your own version of it as Scenario A.
- 3Test one change. Start with load (watts), the input with the biggest effect here: moving load (watts) from 180 to 220 takes the runtime from 5.3 hours to 4.4 hours, a swing of 20% of the worked-example figure.
- 4Check the extremes. At half the example load (watts) (100) the runtime is 9.6 hours; at double (400) it is 2.4 hours.
People also ask
Frequently asked questions
How do you calculate battery runtime?
Runtime (hours) = capacity (Ah) × voltage × usable share ÷ load (watts). Enter usable share of capacity in percent (80 means 80%). At the worked-example inputs the runtime is 4.8 hours.
What does the battery runtime result mean?
Size a battery bank for an RV, a workshop or a backup power supply before you buy it. At the worked-example inputs the runtime is 4.8 hours. It rises with battery capacity (amp-hours), battery voltage and usable share of capacity and falls as load (watts) increases.
How much does battery capacity (amp-hours) change the runtime?
Holding every other input at the worked-example value, moving battery capacity (amp-hours) from 80 to 120 moves the runtime from 3.8 hours to 5.8 hours, a spread of 1.9 hours.
What are the limits of this battery runtime calculator?
Real energy use depends on how the appliance or vehicle is actually run: duty cycle, weather, driving style, battery age and your utility's rate structure, including tiers and time-of-use pricing. Treat the result as a planning estimate and check it against your own bill or trip log. The tables on this page test battery capacity (amp-hours) only from 80 to 120; a value outside that range is not tabulated here.
Which input moves the runtime most in the battery runtime calculator?
Ranked by how far each moves the runtime across the range tested: load (watts) (1.0 hours, 20%), battery capacity (amp-hours) (1.0 hours, 20%), battery voltage (0.8 hours, 17%) and usable share of capacity (0.2 hours, 5.0%).
If I double load (watts) in the battery runtime calculator, does the runtime double?
Doubling it from 200 to 400 takes the runtime from 4.8 hours to 2.4 hours, which is 0.50 times the worked-example figure. So it falls instead of rising. Halving it to 100 gives 9.6 hours.
How much does battery voltage matter in the battery runtime calculator?
The worked example uses 12. With the other inputs left at the worked example, moving battery voltage from 11 to 13 takes the runtime from 4.4 hours to 5.2 hours, a swing of 17% of the worked-example figure.
How much does load (watts) matter in the battery runtime calculator?
The worked example uses 200. With the other inputs left at the worked example, moving load (watts) from 180 to 220 takes the runtime from 5.3 hours to 4.4 hours, a swing of 20% of the worked-example figure.
How much does usable share of capacity matter in the battery runtime calculator?
The worked example uses 80.0%. Holding every other input at its worked-example value, moving usable share of capacity from 78.0% to 82.0% takes the runtime from 4.7 hours to 4.9 hours, a swing of 5.0% of the worked-example figure.
Which inputs change the usable energy (watt-hours) in the battery runtime calculator?
At the worked-example inputs it is 960. Battery capacity (amp-hours) takes it from 864 to 1,056, battery voltage takes it from 880 to 1,040 and usable share of capacity takes it from 936 to 984.
What is a kilowatt-hour?
The energy used by a 1,000 watt device running for one hour. A 100 watt bulb left on for ten hours uses one.
Why is standby power worth counting?
Devices left plugged in draw a few watts around the clock. Across a house of chargers, screens and set-top boxes that adds up, and it runs all 8,760 hours of the year.
Do EV efficiency figures include charging losses?
It depends on how the figure was measured, so check the source. The EV calculator lets you add a loss percentage, so the cost reflects what the wall supplies and not only what reaches the battery.
Sources and evidence
Free Calculators Online is independent and is not affiliated with or endorsed by the source organizations. Educational estimates only.