Device Battery Life Calculator

Use it when your task matches one of these common needs: Estimate how long a power bank may run a tablet, light, router, or camera. Estimate a sensor or small project runtime from battery mAh and average current in mA. It works best when you already know the measurements, amounts, units, or options the page asks for.

In plain language: Current method: usable mAh = mAh * usable capacity percent / 100, then runtime hours = usable mAh / average load current in mA. Power method: watt-hours = (mAh / 1,000) * volts, usable Wh = watt-hours * efficiency / 100, then runtime hours = usable Wh / device watts. The examples on the page are there so you can compare your inputs with a worked example before copying the answer.

Battery capacity mAh: The milliamp-hour rating from the battery, phone, power bank, or small electronics label. Voltage: The nominal battery voltage used to convert capacity into watt-hours. Use the pack or cell voltage from the spec sheet. Device watts: The average power draw of the device while it is running. If you only know amps, multiply volts by amps to estimate watts. Average load current mA: The average current drawn from the battery. For a device that sleeps and wakes, use its duty-cycle average rather than its brief peak current. Efficiency or usable capacity %: The share of labeled energy or capacity you expect to use after conversion loss, heat, battery overhead, aging, and safety cutoffs.

Read the headline answer, then check the smaller lines beside it. For everyday tools, those lines usually show the distance, time, cost, units, or setting that made the answer change.

Real battery life depends on battery age, temperature, chemistry, discharge rate, screen brightness, radio use, power spikes, voltage-converter loss, inverter loss, cable loss, low-battery cutoff, and manufacturer limits. Also check the unit, scale, mode, and result limit because small input changes can change the answer.

mAh alone does not tell total energy unless voltage is known. A 10,000 mAh battery at 3.7 V stores different energy than 10,000 mAh at 12 V.

Use 80% to 90% for many USB power bank estimates. Use lower values when voltage conversion, heat, old batteries, or long cables waste more energy.

Yes. Use the mAh and nominal voltage listed for the pack. Many power banks advertise mAh at the internal cell voltage, often around 3.7 V, not the 5 V USB output.

Power banks often list mAh at the internal cell voltage, then lose energy while boosting voltage for USB output. Phone battery health, cable loss, heat, screen use, and charging overhead can reduce the real number of charges.

No. mAh is a capacity label, not a runtime promise. Voltage, device watts, efficiency, and real usage decide how long the battery may run.

High loads can create heat, voltage sag, converter losses, and early cutoff. A device with startup spikes or bright screens may draw more than its average watt rating.

Use Load current (mA) when the battery and average device current are both described in mAh and mA. Use Power draw (W) when you know battery voltage and average device watts, especially when voltage conversion is involved.

Only when the batteries use the same voltage. For different voltages, compare watt-hours because watt-hours describe stored energy more directly.

No. Phones and laptops change power draw constantly as the screen, processor, radios, charging circuits, and battery health change. Treat the result as a planning estimate.

Divide usable battery capacity in mAh by average load current in mA. A 2,400 mAh battery at 85% usable capacity provides 2,040 usable mAh; at a 20 mA average load, that is 102 hours.

Use average current for runtime. Peak current still matters for checking whether the battery and regulator can support short bursts, but entering a brief peak as if it were continuous will underestimate runtime.

Calculate a time-weighted average first. Multiply each state current by the fraction of time spent in that state, add the results, and enter that average battery-side current.

No. The calculator runs in your browser tab. Your recent answers stay only on the page while you use it, and they are not sent to a server.