Battery life depends on two numbers: how much energy the battery can still store and how fast the laptop uses it. Both can be found — the first from the battery report, the second from a USB-C power meter or the operating system's power readings. This calculator combines them into a runtime and shows how it changes with load.
How to use it
- Enter the battery's design capacity in Wh (from the label or the battery report).
- Enter its health: full-charge capacity divided by design capacity.
- Enter the average power draw in watts.
- Optionally adjust the reserve the laptop keeps before shutting down.
The formula
Runtime (hours) = Wh × health × (1 − reserve) ÷ average watts
The result is an estimate. Real power draw changes constantly, and batteries deliver slightly less energy at high loads and low temperatures.
Typical power draw
Rough ranges for laptops at the wall, screen on:
| Use | Thin office laptop | Gaming laptop |
|---|---|---|
| Idle, low brightness | 3–6 W | 10–20 W |
| Web and office | 6–12 W | 15–30 W |
| Video playback | 6–10 W | 15–25 W |
| Heavy load | 25–45 W | 100 W and more |
These are broad typical figures; measure your own laptop for a real answer.
Worked example: "the battery only lasts an hour"
A laptop with a 56 Wh battery at 85 % health has about 45 Wh usable after a 5 % reserve. At a typical 8 W office load it should run for about 5.5 hours. If it actually lasts one hour, it is drawing about 45 W — far more than normal. That points away from the battery and towards something consuming power at idle: a stuck process, a dedicated GPU that never sleeps, or a leaky component on the board. Those causes are covered in Laptop Running Hot at Idle: Motherboard Causes.
If instead the laptop draws a normal 8 W and still dies early, the battery's real capacity is lower than its report claims, or a cell group has failed. Testing packs is covered in How to Test a Laptop Battery with a Multimeter Safely.
Where it matters
- Telling a worn battery from a laptop that uses too much power.
- Setting expectations after a battery replacement.
- Comparing batteries in mAh to Wh Converter for Laptop Batteries terms.
FAQ
Where do I find battery health?
Most operating systems produce a battery report showing design and full-charge capacity. Some laptops show health in the BIOS.
Why does my laptop's own estimate jump around?
It extrapolates from the last few seconds of power draw, which changes constantly. Averages over minutes are more reliable.
Does screen brightness matter much?
On thin laptops, yes — the backlight can be a large share of idle power.