Monday, September 25, 2023


RVelectricity: More on how SoftStartRV works

To be transparent, NetworkRV, the manufacturer of the SoftStartRV, is a sponsor of

By Mike Sokol

Dear Readers,
Just in time for the long, hot summer, I’ve hooked up my Dometic Penguin II air conditioner on my test bench (literally) to begin gathering data on the peak current draw of a Dometic Penguin II air conditioner with a stock starting capacitor compared to the same air conditioner with the SoftStartRV modification. Watch my video explanation of how their technology works HERE.

The reason for my interest in this product is that I get emails every week from readers wanting to know if they can run a rooftop air conditioner from a 2,000-watt inverter generator, or maybe two air conditioners from a 3,000-watt generator or a 30-amp pedestal. The manufacturer says that their SoftStartRV product will allow you to do that. But you know me and my measuring(!). So I’ve spent the last two days figuring out how to measure and graph an electrical event that takes less than 1/2-second to occur.

Here’s an actual picture of my first meter experiments this week so I could determine what technology could log this much data rapidly. More on that next week, but this took some serious head-scratching to make it happen. Yes, that’s a lot of meters, and no, that’s not all of them as there are lots more brands and models on my bench. But these were the initial contenders.

So I’ll detail the measurement process then do an in-depth article in my next RVelectricity Newsletter due out on Sunday, May 31. But in the meantime I want to show you the basic graph I created from my actual testing. No, this isn’t anything I cut and pasted from marketing literature. This is raw data that I gathered myself and imported into Excel without any editing. Much more on how I did it later, but for now I’m going to explain the basic idea of what the SoftStartRV does in layman’s terms. And it does appear to work exactly as advertised.

As you probably know, all AC motors have a starting surge current that’s much larger than their normal running current once they get up to speed. And that surge current is what trips a lot of generators when you’re running them close to the edge of capacity. As you can see from the diagram, the Penguin II required 35.7 peak amperes of starting current to get the compressor up to speed, which is way too high for a small generator to supply most of the time.

But after I installed the SoftStartRV kit on the exact same air conditioner and measured it exactly the same way, you can see that the peak current was only 17.7 amperes, which is a 50% reduction. So it appears that the SoftStartRV lowered the peak starting current to 1/2 of the factory current by spreading it out over a starting interval that’s twice as long.

Stay tuned for a more in-depth article on this interesting technology in my next RVelectricity Newsletter in a week (sign up here). But in the meantime, know that I’m busy gathering even more data to help keep you safe and cool this summer.

Let’s play safe (and cool) out there….

Oh, to read more about the SoftStartRV and learn where to order one or more, read Chuck Woodbury’s ( editor) article about it here.

Mike Sokol is an electrical and professional sound expert with 50+ years in the industry. His excellent book RV Electrical Safety is available at For more info on Mike’s qualifications as an electrical expert, click here.

For information on how to support RVelectricity and No~Shock~Zone articles, seminars and videos, please click the I Like Mike Campaign.



  1. Does the longer starting time hurt the compressor? Are they not designed to start and run within a period of time so they don’t heat up too much at the starting point?

    • After discussing the physics of soft-starting technology with a number of engineers, I’ve come to this understanding. Because there’s no head pressure in a modern compresser due to the restart delay, and not a lot of inertial mass to overcome on startup, the starting current can be “slowly” ramped up over a few line cycles rather than hitting it hard with a traditional capacitor. The reason manufacturers continue to use hard-start capacitors is simply cost and the fact that most of the time they work just fine. But there is a hypothesis among me and my engineering colleagues that soft-start technology might actually reduce the startup stress on air conditioner compressors since there’s less impact on the components. But until someone wants to do a side-by-side test running both technologies for thousands of hours we may never know, except by anecdotal data from the field about how soon and how many RV air conditioners fail with hard-start capacitors vs. a soft-start controller.

  2. I’m looking forward to the day when a manufacturer that truly cares about customers starts putting newer technology like this into its product. We are still rookie newbies to the RV lifestyle (about three years now) so our education continues. By the time we are ready to trade up, maybe someone will offer something with an A/C already using soft start technology, an inverter generator, twelve volt refrigerator, and HEAVEN FORBID accessible ways to service components. I keep saying that if I knew I wanted to this when I was younger, I would have built my own RV. I wouldn’t have to be a contortionist to winterize my water heater for example. Love RVing, but too old to suffer through the service rituals.

  3. Mike, I have found that when using a generator on any RV that you need a ground/bonding plug to “trick” you energy management system so that you can run any motors without problems. Is this why some of these posters are having trouble starting a A/C with or without a soft starter? I have fixed many of my friends problems with A/C, microwaves etc. by using the bonding plug you recommended. Also the softstart RV does Not have to be cycled 5 times to “teach” the softstart. Where as the other soft start has too….


    • Will, I invented the generator bonding plug over 10 years ago, so I know it has nothing to do with tricking anything. This is how all pedestal power is wired, as well as any installed generators. And it doesn’t have anything to do with peak startup currents tripping the generator. That’s the limits of the overcurrent protection in the generator.

  4. Hi Mike,
    While I understand the technology of soft starting and running two AC units on a 30A circuit, I think most people misunderstand the sustained current rating of a 30A breaker. Most AC units operate at about 12A so two of them would mean the draw steady at 24A while the compressors are running. That is now very close to the sustainable current of a 30A breaker. At this level, it would not take much additional load to pop the breaker.
    I just wanted to point out that while operating two AC units is possible, it is not really practical or sustainable.

    Vic Whitmore
    40E Mandalay 2006

    • I have readers who will gladly leave everything else off if they can beat the heat with a second alternator.

  5. First: I love the research and REAL data… as you so bluntly put it, this isn’t brochure BS… 😀

    That said, I have to headscratch at YOU saying “…how to measure and graph an electrical event that takes less than 1/2-second to occur.” As a career sound guy and (I think you said) electrical engineer, how can you not have a basic oscilloscope? Instead of a pile of meters, an O-scope on a shunt would give you nice, automatic-trigger graphs with many more than 5 datapoints in the event… As for those graphs, it would be clearer to plot before and after on the same scale (yes, I can see the axis is 50%, but still…).

    Thanks for the good work…

    • I have half a dozen scopes including a Siglent digital storage scope, and several current transformers. But remember we’re measuring AC not DC current. So you don’t have a continuous current plot, but rather a series of sine waves at 60 Hz. And unfortunately that creates a sine wave envelope with plus and minus traces that you then have to trigger on and store since this isn’t a repeating event. Add to this we need to do RMS current calculations and not just peak readings due to power factors. Yikes… Yes the first thing I did was experiment for a day to get something on a scope that would be meaningful. But the best repeatable data was from a Southwire Bluetooth digital meter fed by a Fluke current transformer sampled at 200 ms intervals. More to experiment with, but I will detail more later.

      • Thanks for the great explanation of the challenge… I may have to clone your experiment just for fun to learn more about my own scope… Even without transition triggering, I believe I can manually trigger and record 100K samples (10 seconds at 10KHz which would be enough resolution for this…?). Peak can definitely be captured even without the full “shape” of the surge — maybe peak could be your trigger?

        • I may use a current transformer with proper load resistor to feed a digital audio I/O interface then will sample the current waveform 96,000 times per second at 24 bits accuracy which is a resolution of 16,777,216. I can record this on my computer for hours, pick the right sample set, load it into Protools and display the waveform however I like. So I don’t have to worry about triggers, I just begin the “audio” recording, run the air conditioner through a few compressor cycles, then take a look at what I captured.

  6. Mike,
    I always stay in parks with 50 amp service, but I wonder if the softstart device would still be a benefit by extending the AC units lifespan due to the reduced spike during the start?

    • That’s a maybe, but there’s been no studies about this that anyone is aware of. I would have to rig up something to cycle a pair of air conditioners on and off every few minutes for months and see what fails. But it is a reasonable hypothesis.

    • If I can monitor this with a high enough sample rate you can see that it works by chopping up the wave into a variable which is ramped up over a few hundred milliseconds. Capacitors (soft-start or otherwise) do exactly the opposite. They start with the highest amount of current then it reduces as they discharge.

  7. OK, I AM impressed! We rarely use a 30 amp connection, but I’m thinking this is good for all the elements of the system that are now not subjected to the low voltage caused by the A/C starting. Might have to think about making this mod…
    Thanks Mike!

  8. Great stuff, Mike, and a FWIW thing!
    I’ll be very interested to see the more detailed report. I love this stuff!
    I recently purchased a 3500 watt inverter generator. It runs most of the trailer and one AC just fine. It does groan for a second when it picks up the load, but no more than you would expect.

    FWIW, I did a bit of experimenting myself with my new generator. I wanted to test the fuel consumption at a “typical” load and test the auto-shut down system. I ran AC #1 and some of the usual stuff for a few minutes. (Except the residential fridge, I didn’t want to chance hurting the compressor.)
    Then I brought AC #2 on line. It picked up the fan start with a modest increase in RPM. A few seconds later, when compressor #2 tried to start, the engine sounded like it was in cardiac arrest, the volt meter went berserk, and then indicated a sudden momentary drop to 74vac. In a split second, the generator did what is was supposed to do ( I thought). Power to the trailer was cut off, both ACs stopped and the engine immediately went to idle speed. I was impressed, then I discovered the 30 amp circuit breaker on the generator never tripped! To my surprise, my 5 year old surge arrester caught the problem first and immediately disconnect from the power source.
    I turned off AC #2, waited about 2-3 minutes and wallah! The surge arrester “clicked” and everything was back to normal, running from the generator.

  9. I have a Westinghouse 2200 watt inverter generator which I purchased for the purpose of using during off grid situations. After purchase, I was curious about its performance with the Coleman AC rooftop unit (13,500?). My MH is a 2017 Holiday Rambler 30 amp with single air conditioner.
    First run time after warm up, I turned the air on and it killed the generator which was in eco mode (low idle). The second attempt was at full power and turned the air fan on with no compressor.
    Worked just fine as expected. After running for several minutes I then set the thermostat to engage the compressor and that worked very well. Over the course of a 1/2 hour I cycled the compressor on and off at 10 minute intervals with no problem. Without the full load of both the fan and the compressor start up I found the 2200 watt generator to be adequate. I did not use eco mode again.
    With that experience, I found the SoftStartRV device of interest. I purchased one but have not installed it yet. The testing by Mr. Sokol has made me more confident in my purchase as I should now be able to use the air in automatic mode also.
    The benefit of this is the onboard 4000 watt Onan gen uses about 1 gal of gasoline per two hours and is still somewhat noisy. My Westinghouse 2200 uses 1.3 gal per 10 to 12 hours (actual, not advertised) and is much quieter. It can also be placed further from the MH for noise purposes. Noise suppression is the main factor when off grid. Also, the inverter generator is more electronic device friendly. Hope this helps.


Please enter your comment!
Please enter your name here

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Sign up for the

RVtravel Newsletter

Sign up and receive 3 FREE RV Checklists: Set-Up, Take-Down and Packing List.