Laptop batteries have two main ratings on them: Volts and Amperes. Because size and weight of laptop batteries is limited when compared to larger batteries such as car batteries, most companies show their ratings with Volts and Milliamperes. One thousand Milliamperes equals 1 Ampere. When buying a battery, select batteries with the most Milliamperes (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 signifies the energy needed to power one watt for one hour. This notebook battery can power 57.60 watts for one hour. If your notebook runs at 20.50 watts, so this laptop battery could power your notebook for 2.8 hours