Short for Ampere Hour, this is an indication of the capacity of the battery of the UPS, and refers to the current that the battery can supply (in Amperes) in an hour. The higher the AH value, the longer the battery can power the system. The AH value should be governed by the expected backup period, and not the load offered by the system. There is little relation between the VA value of the UPS and the AH value of its battery. So a 600 VA UPS need not support a system longer than a 500 VA UPS if both of them use a 7.2AH battery for backup.