If you are going for off- grid solar power system then there should be a back up support to provide electricity during night or when sun light is less. Here comes the role of the battery, an ideal sizing of the battery as per your needs can solve your problem.
The sizing of the battery depends on:
(1) Calculating the total load
Let us understand all this with an example:
Here is the load chart:
Total load is 150 watts + 80 watts + 150 watts
= 380 watts
(2) Calculating the backup required
Now, how many hours of back up is required to run the specified load?
Say, I want 4 hours of back up i.e. when there is no sun light, battery provides the back up to run the load.Multiply the load with the backup hours and get watt-hours,
= 380 watts x 4 hours
= 1520 watt-hours
This is the amount of energy which is needed as an output from the inverter to run the load. The battery output will go to the inverter, the inverter converts dc power into ac power which in turn will run the load (fans, CFLs and fridge).
(3) Battery basics
(i) Depth of discharge:
(4) Calculating the capacity of the battery
Now, going back to the example, the capacity of the battery will be calculated by considering the following points:
Now, the capacity of the battery which can back up the load for 4 hours is:
0.648x = 1520 watt-hours
x = 1520/0.648
= 2345.7 watt-hours
As for the residential purposes, most of batteries have 12 volts, put this value in the equation, we get,
Or Ah = 2345.7/12 = 195.5 Ah
I want C/10 rated battery, so that I can withdraw a maximum current of 19.55 amperes for 10 hours. Therefore, I will look for battery having specifications as: