To calculate the required amperage for backup batteries, determine the total number of watts needed for all the device(s). The required wattage is used to calculate the amps needed for the backup battery voltage.
Amps = Watts / Volts
For example, if you want to use a 12 volt 20 watt light bulb, the amps are determined by dividing the watts (20) by volts (12) to get 1.67 amps. (20/12 = 1.67)