Battery Life / Runtime Calculator

What does this calculator do?

Enter a battery's capacity and a device's power draw to get an instant estimate of how long the battery will last. It accepts capacity either as watt-hours (Wh) directly, or as milliamp-hours (mAh) plus voltage, and it applies a real-world usable-capacity derating factor so the estimate reflects what a battery actually delivers in practice, not its optimistic rated number.

Formula:

1. Convert capacity to watt-hours: if you enter Wh directly, that's used as-is. If you enter mAh, it's converted with Wh = (mAh × V) ÷ 1000.
2. Apply the usable capacity slider: usableWh = capacityWh × (efficiency% ÷ 100). Batteries rarely deliver 100% of their rated capacity in real conditions — voltage sag under load, cell aging, and converter/wiring losses all eat into it. 80-85% is a reasonable everyday estimate; drop it lower for older batteries or lossy power paths, or raise it toward 95-100% for a best-case/rated-spec number.
3. Divide by the device's power draw: runtimeHours = usableWh ÷ devicePowerW.

Worked example: a 5,000 mAh battery at 3.7V is 18.5 Wh. At 85% usable capacity, that's 15.725 Wh available. Powering a 2.5W device gives 15.725 ÷ 2.5 = 6.29 hours — about 6 hours 17 minutes.

How to use it

1. Enter the battery's capacity and choose its unit (mAh or Wh).
2. If you chose mAh, enter the battery's nominal voltage (printed on most cells/packs — Li-ion ≈ 3.7V, LiFePO4 ≈ 3.2V, NiMH ≈ 1.2V, lead-acid ≈ 2.0V per cell).
3. Enter the device's average power draw in watts. Check the device's spec sheet, a USB power meter reading, or V × A if you only have a current draw.
4. Adjust the Usable Capacity slider if you want a more conservative or optimistic estimate.
5. The estimated runtime and a full breakdown table update automatically. Copy the headline runtime or download the breakdown as CSV.

FAQ

Why isn't my real-world runtime the same as the estimate? Actual runtime depends on load variability (bursty vs. steady draw), temperature, battery age/health, and how efficiently the device's power circuitry converts battery voltage to what its components need. The usable-capacity slider is a single adjustable factor standing in for all of that — tune it down for a pessimistic (safer) estimate.

Should I use mAh or Wh? Use whichever matches what's printed on the battery. Small consumer batteries (phones, power banks, RC packs) are usually labeled in mAh with a nominal voltage; larger battery packs, UPS units, and solar/off-grid batteries are often labeled directly in Wh. If both are printed, Wh is the more direct input since it skips the voltage conversion.

How do I find my device's power draw in watts? Check the device's datasheet or rated power (often printed near its charging port or in the manual). If you only have a current rating in milliamps or amps, multiply by the supply voltage: W = A × V. A USB power meter gives the most accurate real-world reading.

What's a good usable-capacity percentage to use? 85% is a solid everyday default for healthy lithium-based batteries. Use 90-95% for a best-case/near-rated estimate, or 60-75% for older, degraded, or heavily loaded batteries (lead-acid packs under high current draw lose usable capacity faster than lithium chemistries).

Privacy

All calculations run entirely in your browser (or via the API if you call it programmatically) — no data is uploaded or stored, since the estimate is based purely on the numbers you enter.