Converting mAh to watt-hours and calculating run time
Battery capacity is commonly listed in milliamp-hours (mAh), which measures charge rather than energy. To calculate run time from power draw in watts, capacity must first be converted to watt-hours (Wh) by multiplying by the battery's nominal voltage and dividing by 1 000. A 10 000 mAh battery at 3.7 V contains 37 Wh of energy.
Run time is then found by dividing watt-hours by watts drawn. Real-world run time is shorter than this ideal figure because no battery is 100% efficient — heat loss, internal resistance, and the cutoff voltage mean typically 80–90% of rated capacity is recoverable under normal load.
Capacity (Wh) = Capacity (mAh) × Voltage (V) / 1000
Converts charge-based capacity to energy-based capacity, which is needed for power calculations.
Run time (hours) = Effective capacity (Wh) / Power draw (W)
Effective capacity is rated Wh multiplied by the efficiency factor (typically 0.80–0.95).