If you add up the total wattage of the devices you will be plugging into the surge protector then that can be used as the number of joules you need. The formula is
Joules = Watts x Time
Since surges are pretty quick if you figure Time = 1sec you are probably fine. Want to be conservative set T = 2 or 3
The wattage is usually on the back of the device near the power cord.