Battery Life Calculator
Battery capacity and device consumption together decide runtime. This calculator does the arithmetic and applies a realistic efficiency factor, because no battery delivers its full rating.
Formula
Hours = (capacity in mAh × voltage ÷ 1000) × 0.85 ÷ load in watts
- mAh × V ÷ 1000 converts charge to watt-hours
- The 0.85 allows for conversion and heat losses
10,000 mAh at 3.7 V is 37 Wh; usable 31.4 Wh at 5 W gives 6.3 hours
The Basic Calculation
Runtime (hours) = Capacity (Ah) ÷ Current draw (A)
A 2,000 mAh battery powering a 200 mA device gives 2.0 ÷ 0.2 = 10 hours in theory.
Watch the units: capacity is usually printed in milliamp-hours while draw may be in amps. Mixing them puts the answer out by a factor of a thousand.
Amp-Hours Versus Watt-Hours
Amp-hours only mean something alongside a voltage. A 5,000 mAh power bank at 3.7 V holds far less energy than a 5,000 mAh battery at 12 V.
Watt-hours = Amp-hours × Volts
Watt-hours are the fair comparison between batteries of different voltages, and the figure airlines use for their carry-on limits.
Why You Never Get the Full Rating
Expect roughly 80–85% of the theoretical figure. Losses come from voltage conversion, internal resistance, and the fact that most chemistries should not be fully discharged.
Cold weather reduces usable capacity substantially, high discharge rates lower effective capacity further (the Peukert effect), and cells lose capacity as they age. A power bank rated 10,000 mAh typically delivers 6,000 to 7,000 mAh to a phone once conversion losses are counted.
Quick Reference
| Battery | Watt-hours | Usable | At load | Runtime |
|---|---|---|---|---|
| 3,000 mAh, 3.7 V | 11.1 Wh | 9.4 Wh | 0.5 W | 18.9 h |
| 5,000 mAh, 3.7 V | 18.5 Wh | 15.7 Wh | 2 W | 7.9 h |
| 10,000 mAh, 3.7 V | 37.0 Wh | 31.4 Wh | 5 W | 6.3 h |
| 100 Ah, 12 V | 1,200 Wh | 1,020 Wh | 50 W | 20.4 h |
Frequently Asked Questions
- How long will my battery last?
- Divide capacity in amp-hours by current draw in amps, then take about 80 to 85 per cent of the result to allow for real-world losses.
- What is the difference between mAh and Wh?
- Milliamp-hours measure charge and only compare fairly at the same voltage. Watt-hours measure energy and compare across any voltage.
- Why does my 10,000 mAh power bank not fully charge my phone twice?
- Voltage conversion from the cell to USB output loses energy, so usable output is typically 6,000 to 7,000 mAh.
- Does cold weather affect battery life?
- Considerably. Low temperatures raise internal resistance and cut usable capacity, which is why phones die quickly in winter and often recover once warm.