As a simple rule, to calculate how long a 12v deep-cycle battery will last with an inverter multiply battery amp-hours (Ah) by 12 to find watt-hours, and divide by the load watts to find run time hours. Finally, multiply run time hours by 95% to account for inverter losses. Introduction to Solar. . The Battery Runtime Calculator is an indispensable tool for anyone using batteries for power supply, be it in RVs, boats, off-grid systems, or even in everyday electronics. This calculator simplifies the process of determining how long a battery will last under specific conditions. For example, a 10Ah lithium iron phosphate battery can discharge for 5 hours at 2A.
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