Laptop Battery have two main ratings on them: Volts and Amperes. Because size and weight of Laptop Battery is limited when compared to larger batteries such as car batteries, most companies show their ratings with Volts and Mill amperes. One thousand Mill amperes equals 1 Ampere. When buying a battery, select batteries with the most Mill amperes (or mAh). Batteries are also rated by Watt-Hours, perhaps the simplest rating of all. This is found by multiplying the Volts and the Amperes together. For example:

14.4 Volts, 4000mAh (Note: 4000mAh is equal to 4.0 Amperes).

14.4 x 4.0 = 57.60 Watt-Hours

Watt-Hours signify the energy needed to power one watt for one hour. This Laptop Battery can power 57.60 watts for one hour. If your laptop runs at 20.50 watts, as an example, this laptop battery could power your laptop for 2.8 hours.