Battery life refers to the amount of time that a laptop or other portable device can remain independent from AC power, powered only by its built-in battery. It is often one of the main considerations when buying a portable computer.
A laptop's battery life is the result of its power consumption (measured in watts,
W) and the battery's energy capacity (measured in watt-hours,
Wh, or sometimes milliampere-hours,
mAh). The battery's capacity in Wh can be divided by the laptop's power consumption to get a rough estimation of the battery life. It's not always precise or even remotely correct, however, since power consumption can vary greatly depending on what the user is currently doing. For most laptops, typical power consumption during light usage can range from 10 to 20 W, while high-powered workstations and gaming laptops can consume over 100 W.
The capacities of different batteries also vary greatly, usually from 40 to 80 Wh, but high-capacity batteries can be rated for more than 100 Wh. Battery life, therefore, can range from a mere 1-2 hours for a high-powered laptop to more than 10 hours for an ultraportable with a high-capacity battery. Some laptops also have the ability to use more than one battery, bringing battery life close to, and in some cases over, the 24-hour barrier.
Regrettably, laptop batteries do not last forever, and will gradually lose their capacity over time. The rate at which this happens depends on a number of factors, not the least of which is the consistency of charging and discharging the battery, and it will happen even if the battery is not used. For some suggestions on how to prolong your battery's lifetime, see the following questions: