How long will your 100ah battery last with a 20Watt light on?

To determine how long a 100Ah battery will last with a 20W light, you first convert the wattage to amps. Using the formula: Amps = Watts / Volts, and assuming a 12V battery, the light draws about 1.67A (20W / 12V). Therefore, a 100Ah battery would last approximately 60 hours (100Ah / 1.67A) under ideal conditions, without accounting for factors such as battery efficiency and depth of discharge.