Battery Capacity Calculator

By Calxy · Calculation methodology

Convert between milliamp-hours and watt-hours at a battery's nominal voltage and estimate runtime for a load, including a usable-capacity allowance.

Printed from Calxy · https://www.calxy.net/electrical/battery-capacity-calculator

Energy18.5000 Wh
mAh

1e-9 – 1000000000000

V

Use the battery's nominal voltage, for example 3.6–3.7 V for a lithium-ion cell.

Energy

18.5000 Wh

  • Kilowatt-hours0.0185 kWh
  • Joules66,600.0000 J

How this was calculated

Wh = mAh × V ÷ 1000 = 5,000 × 3.7 ÷ 1000 = 18.5 Wh.

mAh to Wh

Milliamp-hours measure charge; watt-hours measure energy. To convert, multiply by the battery voltage: Wh = mAh × V ÷ 1000. A 5,000 mAh phone battery at a nominal 3.7 V stores 18.5 Wh.

Watt-hours are the fairer way to compare batteries with different voltages. A 20,000 mAh power bank rated at 3.7 V holds 74 Wh, but delivers fewer mAh at its 5 V USB output because the voltage is converted.

Estimating runtime

Runtime ≈ battery Wh × usable fraction ÷ load in watts. The usable fraction covers converter losses and the fact that batteries stop before they are fully empty. With a 74 Wh battery, 85% usable and a 10 W load, runtime is about 6.3 hours.

High discharge currents, cold temperatures and ageing all shorten runtime, so treat the result as a best-case estimate.

References

Frequently asked questions

Why do I need the voltage to convert mAh to Wh?

mAh is charge, not energy. The same mAh at a higher voltage stores more energy, so voltage is part of the conversion.

What voltage should I use for a lithium-ion battery?

Use the nominal voltage printed on the battery or cell, typically 3.6–3.7 V per cell. Packs list their own nominal voltage.

Can I take a 100 Wh power bank on a plane?

Many airlines use 100 Wh as a limit for carry-on lithium batteries without approval. Check your airline's current rules.

Last updated . Results are estimates for informational purposes only.