Battery capacity is rated in amp-hours. This is a measure of how much amperage can be drawn from a fully charged battery over time until it is discharged (for a 12 volt battery this is when it reaches 10.5v). Then the amps are multiplied by the amount of time taken to get the Ah rating.

As an example of this, if you had a device that pulled 25 amps and you used it for 30 minutes then the amp-hours used up would be 25(amps) x 0.5(hours) = 12.5 amp-hours.

The standard and most widely accepted rating period for deep cycle batteries is 20 hours. This means that the battery was discharged down to 10.5 volts over 20 hours while measuring the total amp hours it supplies.

Sometimes however the time period can differ, in some circumstances knowing the 6 hour amp rating may be more useful and in others the 100 amp hour rating may be used. Due to something called the Peukert effect a battery gives higher amp hours when it is discharged over a longer time period. For instance the rated Capacity for the Trojan T105-RE is 225 at 20 hours and 250 at 100 hours.