Starting capability is defined by the Cold Cranking Amp (CCA) rating. This 30 second discharge test is expressed in Amps and was adopted by the Australian Standard (AS2149) several decades ago. The CCA rating test was originally conceived by the Society of Automotive Engineers (SAE).
Capability to support accessories in the absence of charge from the alternator is defined as Reserve Capacity (RC). RC is expressed in minutes and assumes a notional accessory load of 25 amps.
Deep Cycle Batteries
Capacity rating (C) is expressed in Amp Hours (AH) and is the product of amps x hours the battery is capable of supplying. AS2149 requires that AH be calculated for a 20 hour discharge (a 100AH battery would be capable of suppling 5 amps for 20 hours).