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To package the battery needed, you have to decide how many days you want to have backup power. Taking the example 14" stand dc fan 20W to run 12 hours at night and a 16W dc light to run 5 hours at night. Total power a day is 320W = 12V 26.7A.

1. Daily usage of the battery = Total power usage / 12V = 320W / 12V = 26.7A. Take note this value is only valid for sunny days.

2. To have 3 days backup battery = 26.7A x 3 days = 80ah.

3. Battery cannot be 100% discharge othewise it may be permanently damaged. The charge controller will protect the battery by reserving 10% of power and cut off the load at 11V. The battery will have to size up 10% more in practical = 80ah x 1.1 = 88ah.

4. The deep cycle battery is recommended to discahrge only 50% of the power stored. Over discharge the battery will shorten the life time. Normal car battery is not recommended to use in solar system because they are designed for only 10% discharge.

See the next page on how to select the charge controller.

How to Calculate the Batteries needed

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