Tripp Lite’s buying guide suggests at least 1,000 joules for costly or sensitive equipment such as computers, displays, printers and audio and video gear. Eaton, which owns Tripp Lite, and Schneider Electric both caution against comparing surge protectors by their joule ratings.
Check that it’s a surge protector first
Before you compare joules, check the label or box for UL 1449. See how to tell a power strip from a surge protector.
What the joule rating means
A joule is a unit of energy. A surge protector’s joule rating is the total energy it can absorb without failing, whether in one surge or over a series of smaller ones. Tripp Lite’s guide says that, typically, the higher the joule rating, the greater the protection.
The parts inside that absorb surges are usually metal oxide varistors (MOVs).
Why joule ratings are hard to compare
Eaton’s 2022 guide to surge suppression says IEEE, IEC and NEMA don’t recommend using joule ratings to compare surge suppressors, because they can mislead. A joule rating depends on the let-through voltage, the surge current and how long the surge lasts, and each maker may use a different test surge to work it out.
In Eaton’s example, a surge protector on a 120 volt system could use a 150 volt or a 175 volt MOV. The 175 volt MOV has the higher joule rating, but the 150 volt one lets much less voltage through and protects better.
Schneider Electric’s Canadian FAQ says joule ratings were used in the 1980s to measure how well surge protective devices protect the equipment plugged into them. It says UL 1449 then standardized on kA (thousand amp) ratings, because many makers stated abnormally high joule ratings that didn’t reflect how much protection their devices gave.
Surge protector makers still print the rating. APC by Schneider Electric gives its SurgeArrest PE6U2, P8U2 and P11U2 strips surge energy ratings of 1,080, 2,630 and 2,880 joules.