Estimate how long a battery could support a steady electrical load before reaching a conservative usable-capacity limit.
The calculator multiplies the battery's rated amp-hour capacity by a conservative usable-capacity percentage, then divides that result by the continuous amp draw. The default assumptions use 50% usable capacity for lead-acid or AGM batteries and 80% for lithium batteries. Your battery manufacturer's limits take priority.
Use the total continuous draw of the equipment that will operate at the same time. Include navigation electronics, pumps, lights and other steady loads. Motors, inverters and devices with changing demand require a more detailed energy calculation.
Battery age, temperature, wiring losses, discharge rate and variable equipment loads can reduce real runtime. Starting batteries also should not be treated like deep-cycle house batteries. Keep a safety reserve and never use this estimate as the only power plan for critical navigation, communications or bilge equipment.
For next steps, read how to test a boat battery, review safe charging guidance, or use the boat solar calculator.
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