What unit of measure is used to describe the amount of work a surge suppressor can do before it stops protecting from electrical surge?

There are 3 items that can be used for surge suppression measurements. Most of the plug in strips use Joules as a measure but this can be very misleading. Joules is a measure of energy. Energy is current (amps) times voltage (volts) times time (seconds) or E=I*V*t.

Voltage during a surge is critical in determining if downstream equipment is protected. So low voltage rating are important. Current during a surge is important as this is a measure of how many amps are flowing diverting the surge from the downstream loads. So high surge current rating are important. Time is not generally critical, but is determined by the characteristics of the surge. For standard surge testing, the time is identical for the same waveshape.

So Joules appears useful at first glance because it incorporates voltage and current in one measure. However, one can increase the joule rating by increasing the voltage (bad) while decreasing the current (also bad). Joules can also be manipulated by using waveforms that are not standardly used in the industry.

So rather than use Joules, it is much better to look at the voltage during a surge and the maximum surge current a product uses and ensure the testing is with a recognized standard surge waveform.