From Kris Arndt:
This is a simple tool to help calculate the estimated run-time* of your battery or battery bank under a given load. By entering the Rated Battery Capacity, measured in amp hours (symbol: Ah) and applied load i.e. Discharge Rate, measured in amperage or amps for short (symbol: A), you can calculate the Actual Battery Capacity at that load plus how much time it takes to discharge to the batteries to safe levels 25% - 50%, (green and yellow) and to the levels at which you may be damaging the battery (orange, red, deep red).The calculator also takes into consideration PEUKERT'S Law.If you happen to have the manufacturers Peukerts Number then you can enter it.Otherwise it's set to 1.1For a leadacid battery, the value of is typically between 1.1 and 1.3.For VRSLAB AGM batteries, it generally ranges from 1.05 to 1.15.For GEL batteries from 1.1 to 1.25.For Flooded batteries commonly known as sealed batteries, 1.2 to 1.6.* Due to factors such as age, make and temperature please note that run-times are estimates.