Battery Energy Calculator
Calculate battery energy in watt-hours and kWh from voltage and capacity.
Estimate runtime at any load for phones, laptops, solar storage, and off-grid use.
Battery capacity is measured in two complementary units: milliampere-hours (mAh) and watt-hours (Wh). Understanding both — and the relationship between them — is essential for estimating device runtime, comparing batteries across voltages, and sizing power banks or backup systems.
Core conversion formula: Wh = mAh × Voltage / 1,000 mAh = Wh × 1,000 / Voltage
Runtime formula: Runtime (hours) = Battery Capacity (mAh) × Efficiency / Device Current Draw (mA)
Where Efficiency accounts for real-world losses (heat, regulation inefficiency): typically 0.80–0.90 for lithium-ion.
Why Wh matters more than mAh for cross-device comparisons: A 10,000 mAh battery at 3.7V stores: 10,000 × 3.7 / 1,000 = 37 Wh A 5,000 mAh battery at 7.4V stores: 5,000 × 7.4 / 1,000 = 37 Wh — identical energy.
Common battery specifications:
| Device | Typical Capacity | Voltage |
|---|---|---|
| Smartphone | 3,000–5,000 mAh | 3.7V |
| Laptop | 40–100 Wh | 11.1–15.4V |
| Power bank (10,000 mAh) | 37 Wh nominal | 3.7V |
| AA alkaline battery | ~3,000 mAh | 1.5V |
| EV (Tesla Model 3 SR) | 54,000 Wh (54 kWh) | 350V |
Worked example: A smartphone with a 4,500 mAh / 3.85V battery powering a screen drawing 350 mA average: Runtime = 4,500 × 0.85 / 350 = 10.9 hours of screen-on time
Power bank efficiency loss: Charging a phone from a 10,000 mAh power bank: Usable energy = 10,000 × 0.85 = 8,500 mAh effective (internal conversion losses) Phone receives ~8,500 mAh worth — equivalent to about 1.7 full charges of a 5,000 mAh phone battery.
Airline regulations: Carry-on limit is 100 Wh without approval, 160 Wh with airline approval. Calculate Wh before flying with power banks or spare laptop batteries.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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