How this instrument works
Energy equals power multiplied by time, so rearranged, time equals energy divided by power. Applied to a battery, that means estimated runtime in hours is simply the battery's capacity in watt-hours divided by the device's power draw in watts — the units are built to cancel out cleanly, since watt-hours divided by watts leaves hours.
This is a theoretical, nameplate estimate. It assumes the device draws a constant, steady amount of power the whole time and that the full rated battery capacity is usable, and neither assumption holds perfectly in the real world. Actual draw often varies with activity — a phone's screen and radios draw far more when active than idle — and batteries rarely deliver every last watt-hour of their rating before performance drops or the device shuts down. Treat the result here as a best-case ceiling, not a guarantee.
It's a common calculation for sizing power banks and backup batteries against expected loads, estimating how long a laptop, phone or drone will run on a given charge, and planning off-grid or solar battery banks against the devices they need to power overnight or through a cloudy stretch.
- Enter Battery capacity (Wh) — read it from the datasheet, or compute it first from amp-hours and voltage with this site's battery capacity calculator.
- Enter Power draw (W) — the device's typical power consumption while running.
- Read Estimated battery life (hours) — how long the battery should power that device at a constant draw.
- Run the calculation twice, once with the device's peak power draw and once with its idle draw, to get a realistic best-case and worst-case runtime range instead of a single number.
Worked example — a 120 Wh battery running a 15 W device
Enter 120 into Battery capacity (Wh) and 15 into Power draw (W) — a small router or similar 15 W device running off a 120 Wh battery. Estimated battery life reads 8.000 hours: 120 Wh ÷ 15 W = 8 hours of runtime before the battery is theoretically depleted.
Questions
Why doesn't my device's actual runtime match this number?
Because real power draw rarely stays constant — screen brightness, radio activity, and processor load all push a device's wattage up and down throughout use, and batteries rarely deliver 100% of their rated capacity before cutting off or degrading. This calculator gives the best-case, constant-draw estimate; real runtime is typically somewhat lower, especially under variable or high-intensity use.
How do I find my device's power draw in watts?
Check the device's datasheet or nameplate for a rated wattage, use an inline USB or wall power meter to measure actual draw directly, or multiply the device's operating voltage by its current draw in amps if only those two figures are listed — volts times amps gives watts.
What if I only know my battery's amp-hours and voltage, not its watt-hours?
Multiply amp-hours by voltage first, or use this site's battery capacity calculator, to get watt-hours, then divide by power draw here. For example, a 100 Ah battery at 3.7 V works out to 370 Wh, and running that at a 5 W draw gives an estimated 74 hours of runtime.
Does a higher-wattage device really need a proportionally bigger battery?
Yes — because life equals capacity divided by power draw, doubling the power draw for the same battery exactly halves the runtime, and doubling the battery's capacity for the same device exactly doubles the runtime. The relationship is directly proportional in both directions, which makes it easy to scale a battery choice to a known device load.
Can I use this for sizing a solar or off-grid battery bank?
Yes, the same division works for battery banks: sum the power draw of every device you intend to run at once (or use their combined average watts), then divide that total into the bank's watt-hour capacity to estimate how long it will last through the night or a cloudy stretch before recharging.