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Old 12-05-2011, 09:30 AM   #1
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Generator - buy one with inverter or not?


Hi, I'm researching generators after two 3 day plus power outages in 3 months and I'm stuck on the option of getting one with an inverter or not.
I understand they claim the power output is cleaner but not sure how important that is since I'm reading conflicting information.
Based on what I want to run I'm looking at 7000 to 8000 watts with a 9-10K peak and plan to use an interlock kit to switch the whole panel over to gen power. I know I won't have full power so we won't be doing laundry and baking muffins but I don't want to limit my power to a handful of circuits hence transfering the whole panel over.
Most of the stuff probably wouldn't be too fussy about clean or not but there are a few pieces of equipment I'm not sure of. In particular the well controller which is a SubDrive box that converts the single phase power to 3 phase. But there are other things like the fridge which is also has digital controls. I'd hate to fry anything but don't want to waste money on a genny feature I might not need.
Plus I'm reading that line power isn't all that clean anyways and that larger genny's tend to put out more stable and cleaner power.
I know rotating electric like a generator natually produces AC but see varying reports of good versus bad output. Most of the ones I'm seeing in the size I'm looking for say they have AVR but no inverter and some even go so far as to say low THD but of course they don't define low.
I'm confused now to what I really need but want to get something in place before mother nature takes another pot shot at New England.
Any advice would be helpful and thanks for reading.

Rick

Last edited by Apollo67; 12-05-2011 at 12:04 PM.
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Old 12-05-2011, 03:33 PM   #2
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You'll be much better off with a regular non-inverter generator. If you need 7-8kW, inverter generators are FAR more expensive than regular generators. In fact, I'm not sure there even are any 8kW inverter generators. Regardless, inverter generators have three advantages over regular ones: Perfect frequency stability, better voltage regulation, and better efficiency at light loads. They also have disadvantages: Poor power quality (not a sine wave, high harmonics), low surge power capability, delicate electronics, and high cost. Inverter generators do not always play well with power electronics like variable speed drives (your well pump) because of the unusual output waveform. They definitely don't like starting heavy motor loads like air conditioners. Of course, regular generators have a couple issues, too: voltage regulation is not quite as good, and the frequency can vary by a few Hz. This can make clocks run fast or slow, and is almost guaranteed to upset UPSs for computers - although most have adjustable trip points and can be set to work on a generator just fine. Larger generators generally have better voltage and frequency stability than smaller ones. For a whole-house backup, the balance of these factors strongly favors a regular generator.
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Old 12-05-2011, 06:28 PM   #3
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Thanks for the info. That's more or less the conclusion that I was coming to but it seemed to contradict the parts I read about inverter based units being better designed for electronic loads. I suppose that depends entirely on the type of electronic load though since your mention of the vari speed drive is right on with other info I had come across.
It might also explain why I wasn't seeing any inverter type generators in the sizes I was looking at. I guess this is the rare case of more is not better. It seems most houshold electronics have a pretty wide range of voltages and even frequencies that are acceptable so perhaps I was overthinking it.

Maybe you could provide some insight on another thing that's been nagging at me since I started looking at them. Almost all the ones in the class I am looking at have 1 240vac 30a output. That would seem to limit the effective usefulness to 7200 w continuous and assuming the breakers dont trip immediately the momentary surges over that should be tolerated ok. If that's the case then an 8Kw gen is overkill since it can't really 'use' the extra power to feed the house. I presume the extra could be used off the 120vac outlets on the gen but that defeats the purpose of what I'm trying to accomplish which is to pump everything it makes into my normal panel to use wherever needed.

Am I right in that? Is that the effective useful continuous duty wattage? and equally important, will the breakers tolerate the momentary surges in excess of 7200 w?

Please note that I don't actually think I'll be running at this level but want to make sure I understand the basic limits and principles.

Rick

PS: Wish I found this site ages ago. Can't imagine how much time it might have saved me over the years...

Last edited by Apollo67; 12-05-2011 at 06:49 PM.
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Old 12-05-2011, 09:01 PM   #4
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That's pretty much right, but with one caveat: If a generator has only a 30A 240V receptacle and no other way to connect the output (like, say, a terminal block), then any continuous power rating over 7200W is pure fiction. "Real" generators above that wattage have the appropriate means to connect the output and use the full rated power.
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Old 12-05-2011, 09:21 PM   #5
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I'll have to check out the terminal block option on anything larger. I don't want to overcomplicate my setup and 7K would probably do just fine so that will be a consideration as well.
I'll be back when I have more questions, which I'm sure I will.
Thanks again!
Rick
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Old 12-13-2011, 07:53 PM   #6
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Some of the inverter type generators only have a 3-wire 120V output plug available..I look for a 4-wire 240V locking plug config to connect my generator cordset to the flanged power inlet. Always be safe and ask your local electrical inspector for clarification.
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Old 12-13-2011, 08:15 PM   #7
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How stable the output of any geeny is depends on a few things,
The quality of the regulation system,
And how stable the loads are !
If you have a stable base load of at least 50% of the gennys
capacity, then the output would be reasonably stable.
But if the load is varing wildly then the output of the
genny will also vary wildly.
So using a 7200w genny, I would keep a base
load of at least 3000 to 4000w.
Then it would be safe to use electronic gear.
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