To calculate the time that a battery will power any appliance – you need to know 2 more values Watts (W) and Volts (V) , apart from Ampere ( A).The formula that we will apply is
A= W/V
Let’s use an example of a few appliances like a bulb of 60 W and a refrigerator of 380 W, totaling 440 W running at 220 V of electricity voltage. So A=440/220 = 2 Amperes. Theoretically a 40 Ah battery will run the bulb and the refrigerator for 40Ah/2 Amperes = 50 Hours.
In real life conditions however you cannot drain a battery capacity completely so the time of battery utilization will be lower. The battery condition like its age and how well maintained the battery is – shall also impact the capacity of a battery.