Battery Runtime Calculator
Estimate how long a battery will last from its capacity in mAh or Wh and the device's average power draw.
Your results
Calculation breakdown
- Usable energy
- capacity (Wh) × efficiency
- Runtime
- usable energy ÷ average power draw (W)
Worked example
A 50 Wh laptop battery at 85% usable efficiency with a 10 W average draw lasts about 50 × 0.85 ÷ 10 = 4.25 hours.
Assumptions
- Efficiency accounts for conversion losses and battery ageing, entered as an estimate.
- Assumes a constant average power draw; real usage varies with workload and screen brightness.
How this calculator works
Estimate runtime for a phone, laptop, power bank or portable device from its battery capacity and the device's typical current or power draw. Useful for comparing power banks or planning device use away from power.
Frequently asked questions
How do I find my device's power draw?
Check the device or charger specifications, or look up typical draw figures for similar devices; actual draw varies with screen brightness and workload.
What's the difference between mAh and Wh?
mAh (milliamp-hours) needs a voltage to convert to energy; Wh (watt-hours) is already an energy unit, so this calculator supports both.
Why is real runtime shorter than calculated?
Battery efficiency losses, ageing, and voltage sag under load usually mean real-world runtime is somewhat less than the theoretical figure.