what is a joule rating
1 There are three levels of protection in the UL rating -- 330 V, 400 V and 500 V. Generally, a clamping voltage more than 400 V is too high.  Energy absorption/dissipation - This rating, given in joules, tells you how much energy the surge protector can absorb before it fails. A higher number indicates greater protection. Look for a protector that is at least rated at 200 to 400 joules. For better protection, look for a rating of 600 joules or more.