It depends on the amount of load you put on the battery. For example, if you put 10A of load then it should last about 15 hours. If you put 150A of load it should last about an hour and so on. Roughly speaking, if you want to power 3 lights of 40 watts each and 4 fans of 75 watts each, a 150AH battery should last around 4 hours. If you want a detailed formula to calculate, here it is: 3 Lights of 40 watts each = 3 x 40 = 120 watts 4 fans of 75 watts each = 4 x 75 = 300 watts Total Load = 300 + 120 = 420 watts Voltage = 12V Load on inverter (in Amp) = 420/12 = 35A Capacity = 150AH Estimate hours it should last = 150/35 = 4.2 Hours You may use the time backup calculator to do these calculations easily.