Two standard ratings are used to measure a battery’s storage capacity:

The amp hour rating tells you how much amperage is available when discharged evenly over a 20 hour period. The amp rating is cumulative, so in order to know how many constant amps the battery will output for 20 hours, you must divide the amp hour rating by 20. Example: If a battery has an amp hour rating of 75, dividing by 20 = 3.75. That battery can carry a 3.75 amp load for 20 hours before dropping to 10.5 volts. (10.5 volts is the fully discharged level, at which point the battery must be recharged.) A battery with an amp hour rating of 55 will carry a 2.75 amp load for 20 hours before dropping to 10.5 volts.

LAMP WATT |
AMP HOURS |
BASED ON 10 HOUR USE |
RECOMMENDED AMP HOUR BATTERY |

6 WATT |
0.5 AMP HOURS |
5 HOURS |
12 HOUR CAPACITY |

15 WATT |
1.25 AMP HOURS |
13 HOURS |
33 HOUR CAPACITY |

20 WATT |
1.75 AMP HOURS |
18 HOURS |
33 HOUR CAPACITY |

25 WATT |
2.1 AMP HOURS |
21 HOURS |
33 HOUR CAPACITY |

32 WATT |
2.75 AMP HOURS |
28 HOURS |
55 HOUR CAPACITY |

40 WATT |
3.4 AMP HOURS |
34 HOURS |
55 HOUR CAPACITY |

Reserve minutes is the number of minutes a battery will carry a 25 amp load before dropping to 10.5 volts. (10.5 volts is the fully discharged level, at which point the battery must be recharged.)

Most liquid acid batteries are designed for automotive or farm implement applications. These batteries are designed to deliver very large bursts of current for short periods when starting an engine and then is immediately recharged by the vehicles' alternator. Therefore the battery manufactures do not provide an amp hour rating. Most deep cycle, Gel-Cell and sealed type marines batteries have an amp hour rating that will appear on the battery.