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How Many Joules Is Your Surge Protector Rated For?

How many joules is your surge protector rated for?

That's an important question!

What's a joule?  Does that have anything to do with the family joules?

In a way yes, but it has nothing to do with the family jewels.

What is a joule?  A joule describes the amount of work needed to derive a given unit of heat or energy, and is a part of the International System of Units.

It is named for English physicist James Prescott Joule (1818-1889) and defines what it takes to pass a current of one ampere through a resistance of one ohm for one second.

There, wasn't that easy?

A joule helps to define a watt:  it is the work required to produce one watt for one second.
And to define a volt:  it is the work required to pass an electric charge of one coulomb through an electrical potential difference of one volt.

You might have a surge protector that looks a lot like this.

What you want to know is if your the surge protector you spent money on will protect your stuff from a power surge.

THAT is where the joule comes in!

If your house or system experiences a power surge (voltage spike) is your "whatever" protected?

What a surge protector does is try to limit the amount of voltage supplied to an electrical device during some sort of voltage spike.  It does so by blocking it, or trying to dissipate it by sending it to ground (a short).  Theoretically they can protect against even a lightning strike.

Note:  sometimes people confuse a power strip with a surge protector and a strip provides no surge protection.

Surge protection has to do with clamping voltage (also called let-through voltage ) and THAT has to do with all the protective components inside the surge protector that divert the unwanted energy.

The joules rating of the surge protector has to do with how much energy the MOV-based surge protector  can theoretically absorb.  What?  MOV means metal-oxide varistor which are components that essentially divert electrical current into a particular direction.

The typical surge protector is built using MOVs.  A lower joule number in the MOV-based surge protector indicates that the detector is able to absorb less energy.  The better protectors can exceed a peak rating of 1000 joules.  Trying to absorb a surge larger than it can protect can cause MOV-based protectors to catch on fire.

However, one of the surge protector's jobs is to send energy to ground and even a low-joule-rated protector can send 30 joules to ground for every joule it can absorb.  A surge lasts microseconds, and some protectors have a delay.  So when you buy a surge protector see if it says how long a delay is, because the longer the delay the more current gets to the the device(s) being protected.

One thing home inspectors have been seeing recently are the newer whole-house surge protectors installed directly into the electric panel box.

Here the surge protector is the large, rectangular "thingie" on the upper right.

Can it protect against a lightning strike?  Yes, theoretically, but lightning strikes have variable amounts of energy, and therefore variable joules are produced that the protector is trying to control.

Fun facts regarding joules:

  • A nanojoule (one billionth of a joule) is the amount of kenetic energy produced by a flying mosquito.
  • A kilojoule (one thousand joules) is the amount of solar radiation received by one square yard of earth in one second, during a sunny day.
  • A gigajoule (one billion joules) is the amount of energy produced by one barrel of oil.
  • A terajoule (one trillion joules) - 63 terajoules were released by the explosion of the atomic bomb over Hiroshima.
  • A zettajoule (one sextillion joules) - earth's annual energy consumption is calculated to be 0.5 (one half) ZJ.

So, this was a lot of information about an important subject - SURGE PROTECTORS.

My recommendation:  when you buy a surge protector look for the number of joules it protects (100 or more is considered good).  The higher the number the better the protection (generally speaking).  And also look for what the delay is - how quickly it works.  Obviously, the sooner the better!  Oh, a yottajoule (one septillion) is how much energy it would take to heat all of the water on earth 1 degree Celsius.  That's a lotta joules!  James Prescott Joule would be proud, to be sure.

Oh, when I played college soccer I did use a protector to dissipate a surge of energy around the family jewels.  It fit inside my jock strap, and we called it a "cup."





Jay Markanich Real Estate Inspections, LLC  

Based in Bristow, serving all of Northern Virginia.

Office (703) 330-6388   Cell (703) 585-7560


Comment balloon 23 commentsJay Markanich • February 20 2014 03:12AM


Way and I mean way over my head Jay.  I'm glad some people can understand it but not me.   

Posted by James Dray, Exceptional Agents, Outstanding Results (Fathom Realty) over 6 years ago

You understand a power surge James!  Both in joules, and as regards the family jewels...

Posted by Jay Markanich, Home Inspector - servicing all Northern Virginia (Jay Markanich Real Estate Inspections, LLC) over 6 years ago

All I know and all I need to know is that, when my panel boxes were installed, the electrician asked and I said "Yes", thereby purchasing the addition of a whole house surge protector in each panel box.

So far, so good.  That said, a surge is not as likely in my home as some because I don't keep electricity supplied to parts of my home that I'm not using.  Some things, such as the well pump, heat pump, etc. are necessary.  However, the breaker for the double oven, even cooktop, whirlpool, etc. are "off" when not in use.  It's a pro-active control of electrical usage that saves me many dollars each year. 

I compute dollars saved into boxes of ammunition.  Each $5 saved is a box of 25 rounds.  Or, each $25 saved is a month of membership at the gym. 

Posted by Lenn Harley, Real Estate Broker - Virginia & Maryland (Lenn Harley, Homefinders.com, MD & VA Homes and Real Estate) over 6 years ago

Hi Jay,

I usually tell my clients not to over load the surge protectors and plugs. I usually don't get this involved explaining why. LOL

James is right. Los Alamos we are not.

Have a good day in Bristow.

Best, Clint McKie

Posted by Clint Mckie, Desert Sun Home, Comm. Inspection 1-575-706-5586 (Desert Sun Home, commercial Inspections) over 6 years ago

You said yes smartly Lenn.  I am seeing the protectors more and more on panel boxes.

Clint - me either.  But everyone gets surge protection.  More is better.

Posted by Jay Markanich, Home Inspector - servicing all Northern Virginia (Jay Markanich Real Estate Inspections, LLC) over 6 years ago

Joules, ergs, it's been a long time since I've done all those equations.

Posted by Debbie Gartner, The Flooring Girl & Blog Stylist -Dynamo Marketers (The Flooring Girl) over 6 years ago

Good morning Jay,

I need to go buy a better one today!

I like Lenn's with the savings for more ammo!

Make yourself a great day.

Posted by Raymond E. Camp, Licensed Real Estate Salesperson Greater Rochester (Howard Hanna Real Estate Services) over 6 years ago
Wow, a lot of great information including the comments, funny I was about to comment about putting the surge protection ahead of all load centers inside the meter main distribution. Murphy is going to get me as I have a brand new surge protector to install in the meter main distribution but keep forgetting about it, maybe this weekend I will install the little bugger. Thanks for the info and great post too
Posted by Rod Pierson, Northern California (Results Real Estate Inc) over 6 years ago

Well this was certainly a lot for my brain to take it so I got  hold of Rich and he assured me that we have a surge protector built into our panel... Hard to believe that I have been in the business all these years and had nary a clue about Joules!!!  Great post, thank you... 

Posted by Beth and Richard Witt, Long Island Cash Home Buyer 516-330-6940 (Long Island Cash Home Buyer) over 6 years ago

Jay, Mr Joules Vern today ;)

Another issue with surge protection is that those MOV that are subjected to voltages over their rating may suffer degradation and then become less function or not functional at all. So it is best to buy the highest rated surge protection you can get.

Posted by Donald Hester, NCW Home Inspections, LLC (NCW Home Inspections, LLC) over 6 years ago

I knew Joule was a physicists name...as soon as you explained it. I vaguely remember learning about that in school. 

Thanks for the info Jay!

Posted by Suzanne Otto, Your Montgomery County PA home stager (Six Twenty Designs) over 6 years ago

This reminds me of when i was in the home electronics business and trying to explain slew rate.

Posted by Joe Pryor, REALTOR® - Oklahoma Investment Properties (The Virtual Real Estate Team) over 6 years ago

I have never heard of a "joule."  You just proved that no matter how many years I have shuffled across the planet, I can still learn something new.  Remembering it will now be the trick

Posted by Myrl Jeffcoat, Greater Sacramento Real Estate Agent (GreatWest Realty) over 6 years ago

Good post Jay. I think I understand most of what you said. My years in the automotive business helps. How big a protector does a 2000Sq. foot house with lots of toys need?

Posted by Tom Bailey (Margaret Rudd & Associates Inc.) over 6 years ago

I "aced" Physics in high school, then again in 2nd or 3rd year of college.  Then I forgot all this great stuff!

    Thank you Mr. Joules... RIP.

Posted by Fred Griffin Tallahassee Real Estate, Licensed Florida Real Estate Broker (Fred Griffin Real Estate) over 6 years ago

There are equations, Debbie?

Raymond - it really pays.  And protects!

Thanks Rod.  Be sure to lick everything first, to make sure your connections are sound.

OK, don't do that!

R&B - well, glad to add to the knowledge base!

I know Don.  There is a lot of counter intuitive thinking with these things and it can get confusing.  Do you agree with the 100 joules rec?


Posted by Jay Markanich, Home Inspector - servicing all Northern Virginia (Jay Markanich Real Estate Inspections, LLC) over 6 years ago

Suzanne - there is a lot of vague in my school past, and getting vaguer.

Joe - never heard of it!  I have heard that Sampson slew a lot of Philistines with the jawbone of an ass.  Similar?

Myrl - so, read on dudette!  There's lots out there.  Did you know Jesse James and John Wilkes Booth were in the KGC together? 

Tom - you will have to ask your electrician boy that question!  Just get a good one!

Fred - if you don't need to use the info, it drips out of your head.

Posted by Jay Markanich, Home Inspector - servicing all Northern Virginia (Jay Markanich Real Estate Inspections, LLC) over 6 years ago

Watt joules? My joules are safe in the jewel vault which is protected by the watt-o-meter (whattometer for lay-people)! I am still scratching my head trying to figure watt is the prob other than too many plugs in the surge strip?

Posted by TeamCHI - Complete Home Inspections, Inc., Home Inspectons - Nashville, TN area - 615.661.029 (Complete Home Inspections, Inc.) over 6 years ago

That there is one fine surge protector Michael!  It's mine and a biggie.  It is made for that many plugs, dude, and can handle them!

Posted by Jay Markanich, Home Inspector - servicing all Northern Virginia (Jay Markanich Real Estate Inspections, LLC) over 6 years ago

Very informative!

Posted by Drick Ward Property Management / Broker Assoc, "RealtorDrick" - Experienced Representation (NEPTUNE REALTY) over 6 years ago

Hi Jay, I homeschooled two of my my kids through high school and remember teaching them joule calculations. Thank goodness my brain is too small to remember all that stuff!

Posted by Tom White, Franklin Homes Realty LLC, Franklin TN (Franklin Homes Realty LLC (615) 495-0752 or www.FranklinHomesRealty.com) over 6 years ago

I'm glad you liked it Drick.

When will you go away for good, gamu?

Tom - we are homeschooling our youngest now who is doing chem this year.  No joule calcs, fortunately.

Posted by Jay Markanich, Home Inspector - servicing all Northern Virginia (Jay Markanich Real Estate Inspections, LLC) over 6 years ago

Tom - I may have just reported you by accident, trying to report the jerk above you.  Sorry if I did!  They will understand not to flog you.

Posted by Jay Markanich, Home Inspector - servicing all Northern Virginia (Jay Markanich Real Estate Inspections, LLC) over 6 years ago

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