Formula
Energy is watts times hours. Capacity divides that energy by voltage:
mAh = watts × hours / volts × 1,000 / usable share
A usable share below 100% increases the nameplate capacity. The runtime page answers hours from a battery you already chose.
Assumptions
- The load is steady.
- Voltage matches the capacity rating.
- The usable share is one percent, not a discharge curve.
Worked example
Five watts for 8 hours at 3.7 V, using the full capacity, needs 10,811 mAh.
Input definitions
- Hours wanted
- How long the load must run.
- Usable share
- The percent of nameplate capacity you expect to get back out.
Limitations
This is a planning figure from the numbers you type. It is not a measurement of a network, a camera, a printer, or a power supply.
Questions
How is this different from runtime?
Runtime divides a known battery by the load. This page multiplies the load by the hours and solves for the battery.
Why does a lower usable share raise the result?
If only part of the nameplate is available, the nameplate has to be larger to deliver the same watt-hours.