How long does a 12v 21ah battery last at 15 watts?

To determine how long a 12V 21Ah battery will last at a load of 15 watts, first convert the capacity to watt-hours (Wh): 12V × 21Ah = 252Wh. Next, divide the total watt-hours by the power consumption: 252Wh ÷ 15W = 16.8 hours. Therefore, the battery would last approximately 16.8 hours under a continuous load of 15 watts, assuming 100% efficiency and no other losses.