Battery Quantity Calculator
Calculate how many batteries you need per year.
Enter your devices and battery requirements to plan your annual battery purchases.
Battery quantity and backup power planning requires matching battery capacity to device consumption. Getting this right prevents both under-stocking and costly over-purchasing.
Runtime formula:
Runtime (hours) = Battery Capacity (mAh) / Device Current Draw (mA)
Runtime (hours) = Battery Capacity (Wh) / Device Power (W)
Conversion between units:
Wh = mAh × Voltage / 1000
mAh = Wh × 1000 / Voltage
Worked example — Smoke detector:
- Battery: 9V alkaline, 500 mAh
- Standby current: 10 µA (0.01 mA)
- Runtime: 500 / 0.01 = 50,000 hours = ~5.7 years (explains why they last years on one battery)
Worked example — Wireless mouse:
- 2 × AA batteries = 2 × 1.5V × 2,800 mAh = 8,400 mWh = 8.4 Wh
- Mouse power draw: ~50 mW (active), ~5 mW (idle/sleep)
- Mixed use runtime: 8.4 Wh / 0.025 W = 336 hours active use
Common battery capacities:
| Type | Voltage | Typical Capacity |
|---|---|---|
| AAA alkaline | 1.5V | 1,000–1,200 mAh |
| AA alkaline | 1.5V | 2,700–3,200 mAh |
| C alkaline | 1.5V | 7,500–8,000 mAh |
| D alkaline | 1.5V | 12,000–18,000 mAh |
| 9V alkaline | 9V | 400–600 mAh |
| CR2032 (coin) | 3V | 220–240 mAh |
| 18650 Li-ion | 3.7V | 2,500–3,500 mAh |
Quantity for a power outage kit: Plan for 72 hours of critical device operation. Calculate total Wh needed for flashlights, radios, and emergency devices, then divide by battery Wh to determine quantities.
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.
SuperGlobalCalculator is independently built and maintained. See how we build and verify our calculators.