What this calculator does
Battery runtime answers a practical question for a battery you already own: given its capacity and a load you want to run, how long before it needs recharging. It is the everyday flip side of sizing a new battery bank from scratch, useful whenever the hardware is fixed and the question is simply how long it lasts.
The calculation only uses the usable share of the battery's capacity, not its full rated capacity, since discharging most battery chemistries all the way to zero shortens their working life and, for some chemistries, can damage them outright. Entering the correct depth of discharge for your specific battery keeps the runtime estimate realistic rather than optimistic.
The formula
Multiply the battery's rated capacity in watt-hours by the depth of discharge (as a percentage, converted to a decimal) to get the usable energy. Divide that by the continuous load in watts to get the runtime in hours.
| Term | Meaning |
|---|---|
| Battery capacity | The battery's rated energy storage, in watt-hours. |
| Depth of discharge (DoD) | The share of rated capacity that can safely be drawn down before recharging, as a percentage. |
| Continuous load | The steady power draw of whatever is being run, in watts. |
| Runtime | (Capacity × depth of discharge) ÷ continuous load, in hours. |
The inputs explained
| Field | What to enter |
|---|---|
| Battery capacity (Wh) | The battery's rated capacity in watt-hours, as stated on its label or datasheet. If it is only given in amp-hours, multiply by the battery's nominal voltage first to convert to watt-hours. |
| Depth of discharge (%) | The maximum safe depth of discharge for your specific battery chemistry: commonly around 50% for lead-acid, and often 80% to 90% for lithium (LiFePO4). Check the manufacturer's guidance rather than assuming a figure. |
| Continuous load (W) | The steady, continuous power draw of the device or devices being run, in watts. For a load that varies over time, use its average draw for a rough estimate, or size for its peak draw to be conservative. |
When to use it
Checking how long a portable power station will last
Entering a power bank's rated capacity, a sensible depth of discharge and the wattage of whatever it is running gives a quick estimate of how long it will last before needing a recharge.
Planning for an outage with an existing home battery
Running the household's critical load wattage against the battery already installed shows roughly how many hours of backup are available, which is useful for deciding what to switch off to stretch that runtime further.
Comparing two batteries for the same job
Two batteries with different capacities and chemistries (and therefore different safe depths of discharge) can be compared directly on the runtime they actually deliver for the same load, rather than on rated capacity alone.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How runtime changes with load at a fixed battery capacity and depth of discharge
A fixed 2,400 Wh battery at 80% depth of discharge, across a range of continuous loads.
| Continuous load | Estimated runtime | Usable energy at this depth of discharge |
|---|---|---|
| 50 W | 38.4 hours | 1,920 Wh |
| 100 W | 19.2 hours | 1,920 Wh |
| 150 W | 12.8 hours | 1,920 Wh |
| 200 W | 9.6 hours | 1,920 Wh |
| 300 W | 6.4 hours | 1,920 Wh |
| 500 W | 3.8 hours | 1,920 Wh |
How runtime changes with depth of discharge at a fixed battery capacity and load
A fixed 2,400 Wh battery running a 150 W load, across a range of depth-of-discharge settings.
| Depth of discharge | Estimated runtime | Usable energy at this depth of discharge |
|---|---|---|
| 40% | 6.4 hours | 960 Wh |
| 50% | 8.0 hours | 1,200 Wh |
| 60% | 9.6 hours | 1,440 Wh |
| 70% | 11.2 hours | 1,680 Wh |
| 80% | 12.8 hours | 1,920 Wh |
| 90% | 14.4 hours | 2,160 Wh |
Questions
Why not just use the full rated capacity?
Discharging most battery chemistries all the way to empty shortens their usable life, and for some chemistries, particularly lead-acid, can cause lasting damage. Using the manufacturer-recommended depth of discharge gives a runtime figure that reflects how the battery should actually be used, not its theoretical maximum.
How is this different from the off-grid battery sizing calculator?
This calculator starts from a battery you already have and works out how long it will last. The off-grid battery sizing calculator works the other way around: it starts from a load and a target runtime (expressed as days of autonomy) and works out what capacity battery you would need to buy. They use nearly identical inputs for opposite purposes.
What if my load is not constant?
This calculator assumes a steady, continuous draw. For a load that varies through the day, using its average wattage gives a reasonable estimate of total runtime, while using its peak wattage gives a more conservative, shorter estimate that assumes the worst case throughout.
Does temperature affect actual runtime?
Yes, particularly for lead-acid batteries, which deliver noticeably less usable capacity in cold conditions than their rated figure suggests. This calculator does not adjust for temperature, so treat its output as a starting estimate in mild conditions rather than a guarantee in extreme heat or cold.
To work out what capacity battery you would need to buy for a target runtime instead, see the off-grid battery sizing calculator. To estimate how much a solar array could recharge in a day, use the solar energy yield calculator.