In order to calculate your battery requirement you need to know two things. First, what is my total load or current draw, and second how long (maximum length of time) do I need to maintain the load on battery. Add up all of the current draws of the items in your system. For example you might have a system with (2) M62 Magnalocks drawing 125mA each, (2) TSB-3 Touch Sense Bars drawing 25mA each, (2) DK-26 Digital Keypads drawing 190mA each, and (1) DT-7 seven day timer drawing 160mA all at 24vdc. Your total current draw will be 840mA. You would, therefore, need the BPS-24-2 power supply. If you need to back up this supply for four hours then you could use the chart below. The closest rating above 840mA is the 1A line. Go over to the 4 hour back up time and you will see that you would need 8 AH of batteries to maintain this system for four hours. If you use 12vdc batteries then you will need (2) 12v/8 AH batteries wired in series for this 24vdc supply.