Some protectors are rated in joules, meaning that if a surge exceeds the joule rating, the protector will likely be damaged. The joule rating of protectors reduces with each surge that the device encounters.  It is not an official safety rating of the product.  Zero Surge products do not have a Joule rating because our products do not degrade with use.  The amount of energy that our products can absorb is limited by the wiring in a building.  At some point the wire in the wall will break down before our product would and at that point the circuit is no longer powered.