What Does The Surge Protector Joule Rating Mean?

The simple answer is that a joule measures energy absorption. The higher rating in joules a device measures, the better the device is thought to be. With regards to surge protection, when the Joules rating is high, the surge protector is capable of handling a larger surge in a single event before it needs replacing. Think of it as the life expectancy of the surge protection device.

The prescribed joules rating can be misleading though, because surges range in severity and the MOVs inside the surge protector (MOV stands for Metal Oxide Varistor, a small semiconductor responsible for stopping the surge) will degrade every time a surge is encountered.

Remember, that surges happen daily in an average home. So, a surge device that has been hit by several surges will not be as effective against its joule rating as it would just out of the box.

Consumers looking for the most reliable protectors on the market should aim high when comparing joule ratings.

Where Did The Term Joule Come From?

The term joule was originally named after physicist James Prescott Joule. While there are several technical definitions and formulas associated with calculating joules, in terms of electricity it is the amount of energy required to produce one watt for one second.

James Joules
Photo Credit: Wikipedia

A surge protector joules rating defines how much of a surge a single unit can absorb in a single event. Units are tested under harsh conditions to see the amount of energy in surges they can handle without fail. If a unit fails at a higher rating it will be rated with the measure it previously passed without fail.

This absorbed energy is released after the event occurs and the unit is capable of absorbing its initial capabilities when it is reconnected to electrical equipment and free of the power surge.

When you are considering a unit, you must consider more than just the joules rating, however. In this type of equipment it is the combination of the surge protector joule rating and the clamping voltage that determines the overall quality of a device.

The clamp voltage will let through surges and divert the unwanted energy from the line without directing all of the energy to the absorption system. Units with a high joules measure are not always the best if there is no protective device to directly dissipate energy elsewhere in the protector.

Experts in the joule system recommend investing in a protector with a rating of 200 to 400 joules. Some will even recommend higher protection with ratings of at least 600 joules.

There has been a wide debate over the joule rating as a parameter for comparing surge protectors though. This rating alone does not mean one protector is better than another. MOVs in the unit you choose should not only absorb the surge, it should also divert the spike and redirect it to ground to prevent overloading the unit.

Most of today’s units have indicators letting you know the device is properly grounded. If it is not, then you will not be protected, regardless of the joules rating or clamping voltage.

Other factors determining how many joules is required is based on how many electronic devices the surge protector has to protect. A multi-outlet protector should definitely have a higher rating, enough to protect several items.

You must also consider the devices you are protecting and whether or not other protectors are installed in the electronic device internally. An electronic with surge protection installed on it will not benefit from being attached to a second device. In fact, it may negate any protection that would otherwise be there.

Always choose a reliable brand respected in the industry and use more than just the surge protector joule rating as the parameter of comparison. Once you find an efficient unit you will have the peace of mind you need in the event of an unexpected power surge.

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  1. Barry Nelson says:

    It is confusing when you say joules are defined as “one watt in one second”, which could be watts PER second, when joules are actually defined in terms of watt-seconds, or the energy expended by using one watt FOR one second. In another article you actually say “watts per second”, which is inaccurate. To calculate joules over time you multiply the number of watts by the number of seconds, not divide. So if you have ten watts for ten seconds you would have dissipated 100 joules (10×10) not 1 (10/10). One watt is one joule per second.

  2. That is right, Barry. It is one watt FOR one second. Sometimes in trying to simplify explanations the translation gets mucked. Your input is appreciated.

  3. Some of your information is also misleading. as they should not degrade at each occurance.

    In fact they are so reliable that MOV’s (metal Oxide Varistors) are normally used in many if not most single and 3 phase applications because they can react to overvoltage energy spikes over and over again for years. . In addition, Varistors react to surges (spikes) by decreasing the instantaneous resistance whenever the instantaneous peak rated voltage of the varistor is exceeded, thus relatively speaking, temporarily shorting the energy spike out . Once the energy spike is gone the resistive value resorts to it’s original value. MOV’s must be able to repeat this process over and over and over again for a very long lifetime. As Long as the joules rating of the MOV is not exceeded the MOV will last forever. However, once the peak voltage rating is exceeded and the varistor joules rating is exceeded by the then energy spike the MOV normally is burnt into either a short or an open and must be replaced.

  4. Excellent information. Easy to read and well written. Thank you. I learned a lot – from comments, too.

    However, I have a question. How do I know when a surge protector needs replaced? I understand everything said above, I think, but how do I know when when a surge protector is so worn out (damaged) that is is no longer as effective as it should be BEFORE anything plugged in to it is damaged? That is, is there a tool, or some place I can take (used) surge protectors, to test them for reliability?

    Thank you. in advance, for answers received.

  5. I would love to hear the explanation as to how putting two surge protectors in series “negates any protection that may otherwise be there.”

    As far as I know, this is entirely untrue. It may be that the statement is badly worded and the author is trying to say that the protection is not required, but putting two surge protectors in series most certainly does not cause one or both of them to no longer provide protection.

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