Battery Discharge Calculator
This calculator is used for calculating the amount of time it will take to discharge a 12volt Lead Acid (or SLA) battery. Enter the value for AHr OR Reserve Capacity, The Load in Amps OR the power used in Watts, and the efficiency of the system you are using. The System Efficiency should be in the range of 60-70 percent for an inverter and ATX Power supply type arrangement, or around 90-95 percent for a DC-DC power supply type.

Enter AHr (AHr)
or Reserve Capacity (minutes)

Enter Load (Amps)
or Wattage Required (watts)

System Efficiency (%)


Time, efficiency ignored (hours):
Time, efficiency used (hours):

Note: These times are best case scenario, and do not include data such as current draws from other devices like the vehicle's electrical system, temperatures other than 80 deg F, and that the battery may be weak or not fully charged.
An example (and test) of the formula is to use 40AHr, 180 watts, and 60 for system efficiency. The result is Time, efficiency ignored=2.667 and Time, efficiency used=1.6.