Battery Runtime Calculator

How to estimate battery runtime

1. Choose the capacity unit printed on the battery or power station. For Ah or mAh, also enter the battery’s nominal voltage; use the pack specification rather than a USB outlet voltage.
2. Enter the average total power used by the connected devices in watts. The power station’s maximum output rating is not the power your devices actually consume.
3. Review the usable-energy percentage, then read the estimated operating time. The initial 85% is an illustrative allowance, so change it when you have better information about the energy available to your load.

Capacity, power and usable energy

Watt-hours measure stored energy; watts measure the rate at which devices use it. At a steady 100 W, 1000 Wh represents ten hours before any reduction for losses or unavailable energy.

Amp-hours and milliamp-hours describe charge. Multiply Ah by nominal battery volts to estimate Wh; divide mAh by 1000 first. The same Ah rating can represent different energy at different voltages.

This calculator multiplies nominal energy by one usable percentage before dividing by average watts. That percentage combines the reductions you choose to allow for. If you enter already-measured usable output energy, use 100% to avoid subtracting the same losses twice.

Use representative average power for cycling appliances. For example, 0.6 kWh used over a day averages 25 W, although starting and running power can be much higher. Combined equipment must still fit the battery or power station’s output and startup limits.

Battery runtime examples

A 1000 Wh power station with an 85% usable share has an estimated 850 Wh for the load. At an average 100 W, that gives about 8 hours 30 minutes.

A 100 Ah battery at a nominal 12 V represents 1200 Wh. With the same 85% share and a 100 W load, the simple estimate is about 10 hours 12 minutes.

A 20000 mAh pack at a nominal 3.7 V represents 74 Wh. At 85% usable energy and a 10 W average load, the estimate is about 6 hours 17 minutes. These are entered assumptions, not tested product runtimes.

What can change the estimate?

The calculation assumes the stated starting energy, a representative average load and no incoming charging power. It does not separately model inverter standby draw, temperature, battery aging, changing voltage, protective cutoffs or discharge-rate effects. A single fixed percentage cannot reproduce every battery’s behavior.

Energy capacity does not establish whether equipment can run or start. Check the actual output specifications separately. For lead-acid batteries and UPS equipment, manufacturer discharge curves or measured runtime can be more informative than this simple energy calculation.

Local calculation

This tool calculates from your entries in the browser without adding uploads, saved input history or input-bearing sharing links. The website’s other services remain governed by its privacy information.