Estimate device battery life from usage.
Battery life is stored energy divided by the rate you consume it, both in watt-hours and watts. Two corrections make the estimate honest: state of health, since a battery reporting eighty-eight percent has lost that much of its original capacity, and the reserve you never actually use because nobody runs a laptop to zero. Both shrink the usable energy before the division.
Usable energy
Usable Wh = rated capacity x battery health x (1 - reserve fraction)
Runtime
Runtime hours = usable Wh / average draw in watts
Battery status tools report it directly on most systems, or divide observed percentage drop per hour by 100 and multiply by capacity. Light web use on a laptop is typically 6 to 12 W, sustained gaming 40 W or more.
They are measured on a new battery with the screen dimmed, radios idle and a light scripted workload. Real mixed use commonly lands at half to two thirds of the quoted figure.