Battery Life Calculator
Runtime from capacity and load, with a realistic derating built in.
The math and the derating
hours = capacity (mAh) / load (mA)
2500 mAh at 50 mA is 50 hours on paper. In practice you get 60-80% of that: capacity ratings assume gentle loads and room temperature, voltage sags before the capacity is truly empty, and electronics cut out above 0 V. The headline figure here applies a 70% factor, which matches real devices far better than the ideal number.
What moves the number most
Heavy loads hurt disproportionately (Peukert's effect, worst in alkaline and lead-acid), cold cuts capacity hard, and bursty loads live or die by the sleep current: a device that sleeps at 50 µA and wakes briefly lasts months, one that idles at 5 mA lasts weeks. For mains cost of a device, see the electricity cost calculator.