mAh, current draw and real battery runtime
What a milliamp-hour rating really promises, why devices rarely hit it, and how average current draw and an efficiency factor shape true runtime.
What a milliamp-hour actually measures
A milliamp-hour is a unit of electric charge: a 2000 mAh cell can, in theory, supply 2000 milliamps for one hour, or 200 milliamps for ten hours. It describes how much charge the battery holds, not how much energy, because energy also depends on voltage. That is why two packs with the same mAh but different voltages store different amounts of usable power even though the runtime formula in mAh and mA looks the same.
Why the label is optimistic
Manufacturers rate capacity under gentle, standardised conditions: a slow discharge, a fresh cell and a comfortable room temperature. Real devices rarely match that. Boost and buck converters lose a slice of the energy as heat, higher discharge currents drop the effective capacity through internal resistance, and the cell warms as it works. The efficiency factor in this tool exists to bridge that gap between the datasheet and the workbench.
Average versus peak current
The single biggest input error is entering peak current instead of average. A microcontroller that sleeps at a few microamps and wakes briefly to transmit can average a tiny fraction of its transmit current, so estimating runtime from the peak badly understates it. To get a realistic figure, work out how long the device spends in each state and blend those currents into a weighted average over one full duty cycle.
From runtime to sizing a battery
You can run the formula backwards to size a pack. If you need a device to last a target number of hours at a known average draw, multiply the hours by the current and divide by your efficiency factor to find the minimum capacity. Add headroom for cold weather, aging and safety margin, since a battery loses capacity over hundreds of charge cycles. That turns a rough runtime estimate into a practical purchasing decision.