Saturday, June 10, 2023


RVelectricity: Hard start capacitors vs. SoftStartRV. Big difference!

By Mike Sokol

Dear Mike,
It would be informative to also show a plot of a “hard start “capacitor with both the standard capacitor and the SoftStartRV. That should dispel some rumors that keep popping up of how a $30 hard start capacitor does the same thing as the soft starter. —Jim

Dear Jim,
I’m getting ready to run a HRDL (the High Rate Data Logger I designed) graph using a Supco® Hard Start Capacitor on my Dometic Penguin II 15kBTU air conditioner next week. That should help explain the differences between a hard start capacitor, stock starting capacitor, and soft start controller technologies.

“If wishes were horses, beggars would ride…”

Sadly, no matter how much we all wish it were true, a hard start capacitor won’t help start an air conditioner compressor using a small generator.

I know that these aftermarket soft start controllers cost a lot of money, especially compared to a $10 Supco hard-start capacitor. But they aren’t the same thing at all. Hard start capacitors are really only useful on a capacitor start induction motor connected to low-voltage utility power. So if you have an air compressor out in the barn on a long electrical run from the house, then a hard start capacitor could work. But generators don’t have a voltage sag issue – they have electronic controls to limit the peak current so they don’t destroy themselves.

While even a standard home electrical service panel can muster up a few hundred amperes of peak current to kick an induction motor up to speed, a inverter generator might be able to supply only 2 or 3 times its rated amperage (at most) for a fraction of a second before shutting down. This suggests that a 2,000-watt inverter generator that can output 16 amps of current for a while might be able to provide 30 or 40 amperes of peak current long enough to kick start a stock 13.5kBTU air conditioner compressor, but maybe only on a good day with nothing else turned on in the RV.

Here’s a detailed graph of what’s going on inside of a soft start controller compared to the stock starting capacitor from the factory.

Click to enlarge.

If you zoom in to take an even closer look, you can see HRDL data showing the actual sine wave of the 60 Hz waveform. Note that in the SoftStartRV plot on the lower half of the graph it appears that the waveform is sort of chopped up. And that’s exactly what these soft start technologies do. By rapidly switching the starting current on and off many thousands of times a second, they slowly ramp up the amperage, and keep the generator (or heavily loaded pedestal circuit breaker) from tripping off.

Yes, the RV manufacturers could include this sort of technology in higher-priced air conditioners, and there are a few inverter-powered air conditioners on the market. But once again, it’s all about the money. Since inexpensive air conditioners work just fine on unmetered shore power, there’s not a lot of incentive to up the price of an RV to include a more efficient air conditioner that will run on a smaller generator.

Click to enlarge.

For more fun, here’s my latest video which uses variable speed playback of HRDL data to show how soft start technology works compared to a stock capacitor. Click on the graphic for an expanded look at the starting amperage envelope, or click the link below to watch the video showing how it works in real time and slow-motion. I think it’s a really descriptive way to show how a complex event occurs. I’ll be doing the same sort of data gathering next month for my study of how the Hughes Autoformer interacts with campground power and air conditioner loads at varying voltages. Watch my latest video HERE.

Please stand by for another HRDL graph next week showing how a Supco Hard Start Capacitor works, and why it’s not a good choice for starting your RV air conditioner from a generator.

Let’s play safe out there….

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.



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Tim Lowe
2 years ago


You are right! Everyone is boon docking! We just returned from 4 weeks in the west and only went to shore power for 3 days. But we were in a class C with a very nice Onan. We are now looking at a 5th wheel. What would your recommended power set up be knowing we gotta have A/C. tim lowe, St Simons Island, Ga

Keira B
2 years ago

Thank you for being so scientific in your answer! So refreshing to see some actual data. You even had color graphs as visual aids. Well done.
I wish every writer did actual research on a subject before they posted. Too many articles are just re-posts of long held beliefs.
Thank you!

2 years ago

Ran out of room. There is a big difference in the return policies, though. From Micro-Air’s web site:

“Retun Policy
A 25% restocking fee is required on allowable non-defective product returns up to 60 days from date of purchase. Refunds or exchanges are not available after 60 days. All returns are to ship prepaid via USPS…”

Micro-Air does make their product for non-RV purposes, which could make a home unit last longer.

2 years ago

I’d really like to see the graph differences between SoftStartRV and the Micro-Air 364, as many people have asked for. A question on Amazon had this answer from the seller: “How is this product different from the microair easy start 364? are the features the same? Answer:They have similar features but the benefits are better. SoftStartRV has a 90-free trial, a 2-year free extended warranty, a free install kit, plus free shipping. All these items are extra with EasyStart. And SoftStartRV has a lower price too…”

Today they both are $300 and both have a 2-year warranty included. One advantage of the SoftStartRV could be newer technology. One advantage of the Micro-Air could be proven technology. One has “learning” and one does not. That’s one part I’m interested in; does “learning” really help and the graphs would show that. Another aspect would be a noise graph of the compressor start-up as measured at the compressors themselves.  Does one “bang” less than the other?

2 years ago
Reply to  J J

I respectfully asked about the “learning” aspect earlier this week in comparison to $10 non-learning single-phase softstarts, … so be careful asking why it’s $300 for RVSS, which he admits to being sponsored by.

The answer, as I understand it, is adjusting the “height and width” (maximum allowable amperage and/or duration) of the PWM interaction. I think Mike DID prove that the learning is “real” — there is a difference in the first couple starts on a new RVSS unit. How important that “ideal” tuning is vs. a “pretty good average,” I honestly can’t say. Which is why I asked the question.

RV Staff
2 years ago
Reply to  Wolfe

I bleeped part of your comment, Wolfe, because you laid into Mike for his response to you, after you have constantly attempted to undermine almost everything he posts. He had reached the end of his rope, and told you in a personal message. You didn’t need to take it public. Just sayin’. —Diane at

Charles Davis
2 years ago

I am a soft start customer, have bought two now for different RV’s, I know they do work and in fact I used the first one on a 40’ 5th wheel toy hauler and a large battery bank and solar and it started my 15,000 btu AC and ran it for hours, I was amazed but it worked.
My second one is on My motorhome and likewise it works using only my 4 group 27 6 volt batteries with solar but I could add a couple more batteries to get a longer ac run during the day.
Am I a pitchman for the product, yes, but I do not know anyone involved with the company and I pay retail for them, they work for me and that’s good news for all RV owners, a good product worth the money I think.

2 years ago

Well Mike, you certainly have become quite the “pitchman” for Softstart. Looking at the graph of the standard cap, is the 150 ms spike the cap discharging?

Mike Sokol
2 years ago
Reply to  friz

I’m not a pitchman, I just explain what everyone is asking about, and everyone seems to have issues with air conditioners this summer. I really think that RV manufacturers should offer some sort of SoftStart option on new RVs since the largest load in an RV is typically the air conditioner. Wait until I get data about how the Hughes Autoformer interacts with the campground grid, or If multiple GFCI interaction is causing tripping, or is it possible to improve campground load balancing to reduce campground low voltage brownouts. I’m just looking for important questions that I can answer via testing.

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