To calculate how long the battery will run the load, just measure or calculate the current that the load will draw when running, and divide the battery Amp-Hour (Ah) capacity rating by the load current, to give runtime in hours. This will be the runtime to 100% depth of discharge (DOD) and should be de-rated by 20% to avoid over discharge. In a cyclic application, (meaning an application where the battery is charged and discharged on a regular basis) the battery depth of discharge should be limited to no more than about 80% of maximum, in order to get a cost effective battery cycle life. Typically, equipment batteries (like SLA or Gel batteries) are specified in Ah over a 20 Hour discharge. Large cyclic or traction batteries are often specified in Ah over a 5 Hour discharge.