Stacking Inverter Generators

I appreciate all the comments about inverter generators, and have about overcome my phobia about unnecessary electronics. SteveU’s comments about generators that can be run by other family members also have me thinking. I would like 4-8 kilowatts to charge batteries and run loads, initially for grid backup, but also to reduce the power bill. Ultimately, charcoal fueling would be good.

I like the Harbor Freight 5000 generator. Runs on gasoline and propane, and propane stores really well. One would work, two paralleled would handle most all needs. Remote electric start is a bonus. Unfortunately, they are 120 volts, and I need 240 volts to run the well pump, and to supply the inverter-charger at full output. Does anyone have any experience stacking inverter-generators to get 240 volts from two 120 volt generators? Would they sinc up through a transformer and run with opposite phase?


No that wont work because that have to run in phase. There is no way to do that.

The inverters when you paralell them read one another. You start one generator and it is litterally feeding power to the other. When you fire up the second one, it reads the phase of the power coming in and then syncs to it to match it. Then it ouputs in phase and they will constantly monitor this.

An off grid inverter system can do this though. Where you take two 120 volt 3kW (typically) and combine them to get 2 phased 220 volt. But they have a communication link to make that happen.

You could get single phase by splitting 120 volt to the motor. But this must come from the same 120 volt leg. Not two different generators. But if the motor requires a split phase then it wont work.


Hi Kent I’ll let other more knowledgeable give details and say yes or no.
You are U.S.
As I underderstand it our household 240 is an actual split phase. Meaning the big power plant as an actually balanced three phase and down locally one of these phases are split in the middle to get our additive (completed phase?) 240 supplied.
So I think not possible. They will want to sync-up only as 120. I’d not want to electrically $$$ take the risk of being the proofs-monkey.

Intersting here. Very critical Comsumers Reports trying to get me to credit card commit for another years on-line sign up allowed me a temporary look-see on the latest products ratings.
In the large 240 volt capable inverter-generators they put the Harbor Freight 9500 as a #2 recommended. The biggest Honda, as their #1. It is over 2X the price.
In the next sizing category down (all 120vac - none with 240vac) the Harbor Freight 5000 inverter-generator is in the top four recommended ratings.
They both got rated high for the quality of outputs. Honestly making claimed power ratings. Customers feed back positively on reliability in annual surveys.

I went with the larger 240vac capable 9500 because of a deep well pumps needs.
Steve unruh


Get one of these and be done with it.

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Actually for just running a well pump this might be a better solution. It would come in handy for other stuff too.


Chances are about nil they do it on their own unless the advertise the ability.
The only way to really do it is to use another inverter system like a solar inverter. And that makes a lot more sense if you have batteries and are the planning on using solar and/or char in the future. It does cost more upfront though.


I do what Matt suggested and have a 5000 watt dual fuel Generator for the well pump. I have a 2000 watt inverter for most household electronic needs and an 8000 watt hooked up the the gasifier. I’m not sure if I want to mess around trying to gasify the inverter unless it got real hard to find gasoline.


More details on my quite-possibly-whacko thinking. Our service entrance is about 100 feet from both houses. That’s where the meter and main disconnect are. It’s hanging on the outside wall of a storage shed. It would be convenient (maybe) to put generators in that open shed, and I have most of the hardware to install a “generator” transfer switch in that box. I can put an inverter and batteries at each of the two houses, and solar panels somewhere (I have one vote on the aesthetics committee, and solar panels are ugly). My current plan (and there have been several so far) is to put generator(s) in the shed, run the generator power through the disconnected line to the houses, and charge batteries when the sun doesn’t keep up with consumption. I like the idea of two generators, either of which could supply minimum needs, and both could allow a little hot water and electric cooking when running.

I’m now thinking that two 120 volt generators, paralleled or not, feeding a split secondary transformer might be best. It would be nice if one of the 5000-really-3800 watt generators could start the well pump, but the inverter can, or can help. The transformer will cost money, but would also allow my collector’s item Lister 3500 watt to be used, if I really need exercise.

Ask me tomorrow for tomorrow’s best plan :slightly_smiling_face:


“They” would never let that happen.

Would they?


The chances of gettting 2 120v waves perfectly offset coming from two different generators is approximately zero. A transformer wont help.


Possibly true if stacked, but paralleled at 120 volts feeding a step-up transformer the chances should be near 100%.

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Screenshot 2024-02-14 at 18-05-42 Microsoft Word - Manual - Autotransformer - rev 02 - EN.doc - Manual-Autotransformer-32A-and-100A-EN.pdf

This is the general idea, with one inverter as shown, or with two in parallel at 120 volts.
(figure courtesy of Victron)

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Sean, the inverter generators are made to stack. I could totally see feeding the input of two sincked inverter generators into a transformer. It might interfere with the inverter generators ability to rapidly rev up or down in response to load though. That is a complex beast though with lots of fail points. Honestly a single 240 generator would probably be a better option though from the sounds of it. Charge a small battery to run a small consumer inverter for critical stuff and just turn the bigger generator off between uses. Insert the smart frugal human into the loop instead of expensive electronics.


I like that David , I worked for 35 years in Hydro Operations Dam making electricity with big 100 maga watt generators with the best electronic for automation available. We still had at least two people watching over the systems 24 hrs. / 7 day a week from the first day of operating the generators and putting them on line. Even our smaller fully automation unman power plants had Operators inspections daily because the automation needed to be inspected and watch over. Automation works great until it doesn’t work. Then you need people with knowledge and know how to fix it and operate it.


Good morning KentP.
I want to go back to your described physical service lay out and the bit of what you said on the user human side of it.

Your medium-small (2) portable inverter generators many to be not the best idea for at least three reasons.
You are distanced enough away you do not need the fully enclosed noise isolation. You can get open framed inverter-generators for at least 20% less expensive.

A second thing to seriously consider is theft. Man packable, portable, suffer from a huge theft problem. I actually have mine individually stashed because I am a grid-down, back-up fellow.
Even so I want the much quieter operation of fully sound insulated, and lower RPM running inverter types so’s using I will not be targeted for later theft-searches. We do leave at times taking the dogs with up. Gone overnight. Gone for 2-3 days.

I’ve personally gone from little noisy engine driven DIY, DC generators. To my own 950 pound Listeriod “hopper” engine. Many many problems to overcome then. And the Wife hated it. She would never have ran it.
To a diesel tractor driven 1800rpm brushless power head. Great. Except when ever using it the tractor would be tied up unable to work anything else.
To a V-Twin E.F.I. Miller Trailblazer welder-generator. I never worried as much about anything being four-man drunken team, packed-off and stolen as with that ‘solution’.

To now all sold off; or set aside for 1) Honda 2000; 1) Yamaha 2800; and 1) Harbor Freight 9500. All inverter generator units by my intents. The best compromises I have come up with so far. Actual using wattages of 1600 + 2200 + 7600. Or any combination of these.

The third think it through man:
For the complexity of system you are currently proposing setting up ONLY You will be the one truly understanding it. No matter how you would make it flip-switches; turn knobs operable with instructions placards . . . when it does not work as expected . . . It will be unusable until YOU trace down and fix the why-not(S) of it.
How is your health?
You might as well forget about using a troublesome fuel like wood, or even charcoal.
Actually only a very stable spec fuel like propane or street gas can do flip switch systems for non-tech users.
Not even gasoline or diesel. They age and gel.


My point is you have to have -some- way to sync the wavelength. You can’t just plug two random generators in and expect the waveforms to be synced.

Some inverter generators sync, some don’t. And even if they do it doesn’t mean they can if they are two different brands. Which is fine when you first start, but in say 5 years and you have to replace one of them, then you either have to match it with the other one provided they still make it and use the same protocol, or buy a new set.

Which is back to essentially using a solar inverter (or ones that stack) with a charge controller and if say he runs woodgas with the listor, it is as simple as swapping the plugs.

As a ‘bonus’ a lot of the offgrid inverters can actually use grid power, so if something bad does happen, they can be wired to so it is just a matter of reconnecting to the grid with a phone call.

The waveforms have to be synchronized somehow. whether it is at 0 or 180 degrees, it is the same thing a transformer does not synchronize waves. An inverter converts DC to AC power.

Honestly with the two houses running off the same mains, you are better off to consult with an electrician that can do a look see to see what you have and what would need to be done to keep everything up to code. Almost all of the code is a result of fires and deaths. under and overcurrent situations also easily lead to premature equipment failures.

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Thats way too easy silly. We must over complete this :slight_smile:


I am really grateful for all the suggestions. Many of them are spot on, some aren’t a perfect fit for our needs (and maybe my weaknesses). The transformer I think will work. If I buy two matching inverter-generators, they will surely work paralleled, especially if I were to buy the parallel kit (which I don’t really think is necessary based on Matt’s work). I think it would work stacked also, since the second generator would just see the first generator’s voltage, phase inverted, looking into it’s winding. No point in arguing, since parallel has almost got to work.

The inverter generators should be more efficient at partial load, but the main reason for going that route is what Steve describes. I am mostly healthy, but I am not young. I have a very smart family around me, but not all are electronic tinkerers (like, none), so open the fuel valve and push the button is attractive. As the prepper’s saying goes, two is one and one is none." That’s why two mediums seem a little better than one big one. Theft is not a big issue around here, but times are changing, so quiet and covered seems wise. Steve, I think your big Listeroid is (was?) a marvel, my 6 hp air-cooled is only ~400 pound, plus the 1800 rpm Kohler
generator head. My wife likes robust, but “hold the start button, with the compression release on, let it spin up, close the compression release, release the start button when it makes still more noise and smokes, move the pump control to run, while it bounces around on the shock mounts,” is not a good plan on my part. One open frame inverter-generator would be the same price, roughly, as two quiet ones, with a little more power. I don’t really know about low-load efficiency or gasifying. Anyone have an idea?

The inverter-charger I’d use with this is 4kw, with a big surge capacity. No problem with the well pump, so far, at least in testing. Solar is on the list, and panels stacked in a shed, but there is no perfect place for them, so first the backup with batteries and generator, then the solar.

Maybe I should put in a piston well pump with a hand lever and a jack with a pulley, put a water jacket on the wood stove, and be content. Oh yeah, and candles.


That makes it to easy though. 8 EG4 6000XP paralleled for 48kw total output at the mains give 200amps of 240v which is probably the mains service wiring is rated for. Add a bunch of batteries, then start hooking up generators or solar panels or whatever to keep the batteries up. By this time, you are out of a single cylinder generator, and need multi-cylinders which means you can use a WK gasifier. You get bonus points for using a rusted out dakota with a genhead.

Overcompleted enough or did I miss something besides the empty wallet? :slight_smile:


LOL yeah now we are thinkin. haha