Sunday, March 26, 2023


Surge Protectors and Joules and MOVs, oh my!

Dear Mike,
After reading all your articles about voltage problems, I’m now in the market for a surge protector but a little confused by all these Joule numbers. Some are rated at 1,400 Joules of protection, while others are 2,100 or even 4,200 Joules. So first of all, how do you pronounce Joules, and just what are they? If these gadgets are so important, how come RV manufacturers don’t build surge protectors into their RVs at the factory? —Russ

Dear Russ,
You sure ask a lot of questions, which shows me you’ve been doing your homework. So I’ll answer them in order.

First of all, Joule is pronounced just like Jewel, those sparkly things that cost so much. If you need a playback of how it sounds, go here. And since you asked, each Joule is equal to 1 watt-second of energy. So 1 watt of energy for 10 seconds equals 10 watt-seconds, which equals 10 Joules. Just as 10 watts of energy for 1 second equals 10 watt-seconds or 10 Joules. And 1,000 watts for 1 second equals 1,000 Joules, etc. It’s a linear relationship so basic addition and multiplication is all the math you need.

Next, inside of every surge protector is something called a MOV, which is an acronym for Metal Oxide Varistor. The job of the MOV (pronounced like Mod, from the Mod Squad), is to clamp any dangerous peak voltage surges that might come in your shore power line. It would take at least a few articles to explain all the possible causes of these voltage surges and how to stop them, so for now we’ll just concentrate on the energy absorption aspect of the MOV devices inside a surge protector.

Each MOV has two important ratings. Peak voltage is how high the surge voltage is allowed to get before the MOV goes into action and shorts (clamps) it out. Don’t worry about that one for now. But MOVs are also rated for Joules of energy absorption. You can imagine that a small voltage surge might only cause a small amount of energy to be absorbed when it’s clamped by a MOV, while a large voltage surge would cause a large amount of energy absorption, and you would be right. A standard size MOV device like you find in a typical surge protector is rated for 350 Joules.

So how do the manufacturers get higher Joule ratings? Well, you can put a number of 350 Joule MOV devices in parallel to increase the total amount of energy dissipation. Here’s what the inside of a surge protector with 2100 Joules of surge protection looks like. Note there are six (6) MOV devices inside, each one rated for 350 Joules. A little arithmetic shows us that 6 x 350 = 2,100. Yes, six MOVs times 350 Joules per MOV equals 2,100 Joules total of surge protection in this case.

As far as surge protection from voltage surges go, more Joules are better. That’s because all MOV devices are sacrificial. That is, a MOV rated for 350 Joules can take a single 350 Joule hit from a surge, or it can take ten 35 Joule hits. And after it’s had all the voltage surges it’s rated for, the MOV is now worn out and can’t protect your electrical system from voltage spikes any more. That’s why all quality surge protectors have some sort of indicator light to show that their MOV devices are healthy and the surge protection is still working. But once that light goes out you know it’s time to have the MOV devices replaced by the manufacturer. I’ve looked inside of a bunch of different surge protector products, and there’s just not enough room for a DIY replacement yourself. So always contact the manufacturer about how to get this done.

Finally, the 64 million dollar question. Why don’t RV manufacturers include some sort of surge protection device in their RVs from the factory? Well, it’s all about the money. I’m sure that Surge Guard, Progressive and everyone else would love to put one of their surge protectors in every new RV. But it’s a hard sell because that would increase the cost of every new RV in an already highly competitive market. So unless all of you ask for it, the chance of this sort of thing happening is pretty slim.

But as I continue to educate you all about RV electrical safety maybe they’ll get the message. Until then, you’ll need to purchase your own surge protector. And yes, I think that EVERY RV needs a surge protector. I’ll present more on basic vs. advanced surge protectors in my No~Shock~Zone seminar that will debut in Elkhart at the RVillage Rally on May 19, 2018. And yes, I’ll turn it into an article for you all to read later.

Let’s play safe out there….


Mike Sokol is an electrical and professional sound expert with 40 years in the industry. Visit for more electrical safety tips. His excellent book RV Electrical Safety is available at For more info on Mike’s qualifications as an electrical expert, click here.



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Mike Sokol
4 years ago

I plan to call Progressive Industries to be sure, but I heard that since they were bought out last year they no longer offer a lifetime replacement of the MOVs when they wear out. But that may just be a rumor. I’ll call the company and find out for sure.

4 years ago
Reply to  Mike Sokol

J that would be great info, Mike. Mine has that lifetime warrantee, but it would be good to know if it’s TP now.

Any stats on how often RV surge MOVs need to be replaced on average? I seldom blow up my strips at home, but RVs are more exposed to bad situations.

Roy Ellithorpe
4 years ago

I thought I was understanding, until I started thinking “350 joules is one 100 watt light bulb for 3 1/2 seconds”. To me that or even 6 of those doesn’t seem like much compared to a lightning strike on the power line.

4 years ago

Honestly, I wouldn’t WANT any more standard equipment. The buyer is still paying for everything anyway, so making it standard just makes a certain choice mandatory where I’d rather pick out my own options. This is particularly true where the standard option doesn’t meet my requirements and I end up paying for standard features I KNOW I’m going to tear out immediately.

What I think dealers SHOULD do, perhaps, is provide a list of “strong recommendations” with new units, so buyers will be informed what’s missing before they have a problem. “We included one battery; you’re going to want 4 if you leave resort-campgrounds… We recommend power management; here’s some options or pick your own.”…etc…

4 years ago
Reply to  Wolfe

I agree with you, when you spend a 100M or more you should not have to spend extra money for things that should have been there. With only one year warranty on the coach, some of these problems wont show up until after the warranty has ended. Patrick………………

4 years ago
Reply to  Patrick

100 million dollar rv…. wow.

4 years ago
Reply to  RWB

But…. yeah . Surge protection and peace of mind is worth an extra 300 bucks. I will install it in my 24k toyhauler.

RV Staff
4 years ago
Reply to  RWB

“M” is the Roman numeral for 1,000 — but it can get confusing. —Diane at

Mike Sokol
4 years ago
Reply to  RV Staff

That’s why I always use k from kilo for thousands, as in kilograms, kilometers, kilovolts, and kilobytes. Of course, in computers it’s really 1024 not 1,000, but close enough. So I’ll write kv, kg, km, etc…

4 years ago

Thanks for the very clear write up Mike. I was just wondering about the Joule ratings this week as I needed to replace an old CAMCO 55301 portable that was well beyond warranty and had begun acting strangely. Ended up going with a Progressive EMS-PT30X based on the recommendations of friends, the fact that it’s American made and comes with a lifetime warranty. It was a bit concerning that the older has a 2410 Joule rating and the new one has a 1710 Joule rating. Hopefully, with the lifetime warranty, the company will take care of it if I ever burn through all 1710.

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