How to Calculate Battery Life
Battery life (hours) = usable energy (Wh) รท power draw (W). Usable energy is calculated as: (capacity in mAh รท 1,000) ร voltage (V) ร efficiency (%). For example, a 5,000 mAh phone battery at 3.7V with 85% efficiency has about 15.7 Wh of usable energy. At a power draw of 2.5W, that gives roughly 6 hours and 17 minutes of use.
Typical device power draws: smartphones 2โ4W, tablets 5โ10W, laptops 15โ65W, e-readers 0.3โ1W. Voltage varies by battery type โ standard Li-ion/Li-polymer cells use 3.7V nominal; laptop packs are typically 10.8โ14.8V.
Actual battery life varies with screen brightness, network connectivity, temperature, and battery age. This calculator gives an ideal maximum โ real-world performance is typically 10โ25% lower.
Frequently Asked Questions
Wh = (mAh รท 1,000) ร V ร efficiency (%). Battery life (h) = Wh รท power draw (W).
Use 3.7V for standard Li-ion/Li-polymer cells (smartphones, power banks). Laptop batteries vary from 10.8V to 14.8V โ check the battery label.
Internal resistance, heat generation, and voltage conversion cause energy losses of 10โ20%. Battery aging reduces efficiency further.