Battery Life Estimator
Enter a battery’s capacity, the current your device draws, and an efficiency factor to see how long it should run — in hours and minutes — plus the energy it stores in watt-hours.
From power bank to smart sensor
Battery specs are marketed in mAh, but what you care about is hours — how long the earbuds last on a charge, how many times a power bank refills a phone, how long a door sensor runs before it needs a new cell. Turning capacity and draw into real runtime, with an honest efficiency factor, is the quickest way to compare devices and avoid the dead-battery surprise.
Running mains-powered gear instead? See what it costs to keep on with the Smart-Home Power Cost calculator.
Frequently Asked Questions
How do I estimate a device's current draw?
Check the spec sheet for a rated current in milliamps, or work backwards from power: watts divided by the battery voltage gives amps, which you multiply by 1,000 for mA. For a rough gauge, a small sensor or tracker sips a few mA, a Bluetooth earbud tens of mA, and a bright torch or fan hundreds. If a device already quotes a runtime, you can reverse this tool to sanity-check the draw it implies.
Why isn't runtime just capacity divided by draw?
Because no battery delivers 100% of its rated capacity to the load. Voltage regulation, conversion losses, self-discharge, and the way capacity is measured under ideal lab conditions all shave off usable energy, so this tool applies an efficiency factor — 85% by default — to the ideal figure. Lower it for cheap electronics or cold conditions; raise it toward 90–95% for efficient, well-regulated gear.
What does the watt-hours figure tell me?
Watt-hours (Wh) express the actual energy stored, calculated from capacity and cell voltage, and they're the fairer way to compare batteries — a 10,000 mAh power bank at 3.7 V holds about 37 Wh regardless of how the mAh is marketed. Watt-hours also decide what you can take on a plane: airlines cap carry-on lithium batteries by Wh, typically allowing up to 100 Wh without special approval.
Why does my device last less than the estimate?
Real loads rarely stay constant. Screens brighten, radios wake to sync, motors spike under strain, and batteries lose capacity as they age and in the cold — all of which pull runtime below a steady-load calculation. Treat the number as a best-case ballpark for planning, and build in a margin for anything you actually depend on staying powered.
A steady-load ballpark. Real runtime depends on temperature, cell age, and a load that rarely stays constant — build in a margin for anything you depend on.