Whether you're wondering if your phone will last a long-haul flight, sizing a power bank for a camping trip, or calculating how long a UPS will power your network equipment during a power cut, the underlying maths is the same. Here's how to calculate battery life accurately.
Understanding Battery Capacity Units
- mAh (milliamp-hours): Used for small batteries (phones, tablets, power banks). A 5,000 mAh battery can theoretically deliver 5,000 milliamps for 1 hour, or 500 mA for 10 hours.
- Wh (watt-hours): More universally useful. A 100 Wh battery holds 100 watts of power for 1 hour. Used for laptops, portable power stations, and EV batteries.
- Conversion: Wh = mAh × Voltage ÷ 1,000
Battery life (hours) = Battery capacity (Wh) ÷ Device power consumption (W)
Battery life (hours) = Battery capacity (mAh) ÷ Device current draw (mA)
Apply efficiency factor: Multiply by 0.85 (85% — accounts for conversion losses)
Common Device Power Consumption
| Device | Typical Power | Notes |
|---|---|---|
| Smartphone (screen on) | 2–4 W (600–1000 mA) | Screen-off: 0.5–1 W |
| Tablet | 5–12 W | At screen brightness 50% |
| Laptop (light use) | 15–30 W | Word processing, web |
| Laptop (gaming/intensive) | 60–120 W | Dramatically reduces battery life |
| Wi-Fi router | 5–15 W | UPS sizing |
| LED lamp (10W equiv) | 10 W | Per bulb |
| CPAP machine | 30–60 W | Humidity = more power |
| Portable fridge | 35–60 W | Average, cycles on/off |
Worked Examples
Smartphone: 4,500 mAh Battery
At 700 mA average drain (mixed use): 4,500 ÷ 700 × 0.85 = 5.5 hours screen-on time
At screen-off low power (100 mA): 4,500 ÷ 100 × 0.85 = 38 hours standby
Power Bank: 20,000 mAh @ 5V Charging a Phone
Power bank Wh = 20,000 × 5 ÷ 1,000 = 100 Wh
Phone battery = 4,500 mAh × 3.7V ÷ 1,000 = 16.65 Wh
Charges possible = (100 Wh × 0.85 efficiency) ÷ 16.65 Wh = 5.1 full charges
UPS for Router (10W) and NAS (25W) = 35W
UPS: 600 VA / 360 Wh at 80% load
Runtime = 360 Wh × 0.85 ÷ 35 W = 8.7 hours
Factors That Reduce Battery Life
- Temperature: Cold (below 10°C) reduces capacity by 20–40% for lithium batteries
- Age: Li-ion batteries lose 20% capacity after 300–500 cycles; 80% threshold is common
- Load variability: Peak loads (turning a motor) draw far more than rated average power
- Partial charge: Most power banks are rated at full charge; if at 70%, adjust proportionally
Extending Battery Life
For lithium batteries: keep charge between 20–80% for maximum longevity. Avoid full discharges and 100% charges for daily use. Temperature management is critical — never store or charge in hot cars. For devices you rarely use, store at 50% charge.