A Joule is a measurement of energy (1 joule equals the work needed to produce one watt of power continuously for one second). The joule rating of your surge protector is based on the number of MOVs (metal oxide varistors) inside the protector. A higher number of joules should equate to a higher ability to absorb spike or surge energy. Each MOV has a rating and when you add these all up, you get the total number of joules.