Obtain the amp-hour rating of your energy generation or energy storage device. A boat battery, for example, may list a 0.8 amp-hour rating. This rating could be listed on the side of the product, or in the owner's manual. Instead of amp hours, the rating also could be listed in milliampere hours, often abbreviated mAHr.

To convert milliampere hours to amp hours, divide the milliampere-hour rating by 1,000. For example, If a battery is rated at 3,500 milliampere hours, the amp-hour rating would be 3.5 amp hours, since 3,500 divided by 1,000 is 3.5.