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.