Power is quoted in milliamp hours (mAh), such as 2000mAh. With this example, used with a device that draws 2000mA, the batteries would last for one hour; used with a device that draws 1000mA, the batteries would last for two hours. What you really need to know, is that the higher the number, the more power the batteries hold. How important this is, depends on the type of device that you're using. See the question about the discharge rate (above) for more.