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Features to look for in stand alone generator
I'm looking for a stand alone generator in the 4 or 5000 watt (continuous) range, gas powered. It will be used to run small power tools and for emergency power backup (basically get my furnace going, and keep refridgerator cold).
There is a huge range of prices for these units. What features or construction should I be looking for (i.e. bearing type, brushless, "cleanliness" of the power, etc.)
I've been told that some electronic furnace controllers "choke" on the dirty power from some portable generators.
Thanks for any advice you can give.
John Mc
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Features to look for in stand alone generator
I don't have the furnace yet, since the house is under construction. I'm not so much worried about the power rating (including surge) of the furnace generator combo. I can figur that out. It's more the "dirty" power concern. I've heard some of the "Home Depot" store models put out more of a triangualr waveform (not sawtooth, but triangular) which causes some furnace controls and other electronic equipment to choke.
My heat will be a boiler (probably oil or propane fired) Most of the house is radiant floor heating, with a few small pumps to circulate it. These don't draw anything like a well pump (though I will need to run that... separately if necessary). The controller will switch on and off various zones as needed. I might occasionally need to run other electronics as well.
I see specs on harmonic distortion quoted on some units (i.e. "less than 6% harmonic distortion" but I don't know what's considered a good number.
I don't anticipate running this a lot of hours, so I'm hoping I don't need one of the $1500 - $2000 units. On the other hand, I'd be willling to spend more than the $400 or $500 for the low end units, if I knew I was avoiding some of these problems, or otherwisse getting something for my money.
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Features to look for in stand alone generator
TomG-
From what I read, a lot of the distortion is caused by harmonics of the main sine wave. I know that triangular waveforms basically are a sine wave with a strong third harmonic (so if you are generating 60 Hz, you'd have a strong 180 Hz component superimposed over it). Spikes and surges tend to get filtered out pretty well by an inductor (transformer), but I don't think harmonics do (I may be wrong on this, however).
I've heard from a couple of people who had problems where the controller on their furnace would not start up on some generators. They put it on a scope and saw a triangular waveform. Borrowed their neighbor's generator, and it ran fine... that one scoped more like a true sine wave.
I'm concerned about that factor, but also about getting a decent basic unit. Unfortunately, I don't know much a bout the mechanical aspects (why is OHV touted as superior on a small gas engine? Is it worth paying extra for? Who makes good engines: Briggs, Tecumseh, Northstar?) What features should I be looking for on the generator half?
John
John
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Features to look for in stand alone generator
Steve-
Unless you spend big bucks for a "true sine wave" inverter, most inverters put out a square wave or a "modified sine wave" (which is basically a square wave with slightly sloping sides). I would not have thought most AC generators used an inverter. Inverters are used to transform DC to AC. Since there are losses in the conversion, I would figure they'd just generate AC directly, rather than generate DC and convert to AC.
(They only reason I know anything about inverters is because I've been reading up for possible addition of solar &/or wind power to our new home... requires inverting the DC power to AC for running traditional appliances.)
John Mc
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Features to look for in stand alone generator
TomG-
Re: generator hook ups to utility equipment
My general contractor has an electrician who is familiar with this, as far as meeting the applicable codes. If you have some suggestions beyond that, I'd be very interested. I'm thinking of things the code doesn't deal with but that someone who has lived through an installation may have good ideas about. Are there things you wish you had done differently? Are there different types of subpanels or transfer switching that is more convenient or more flexible for changes down the road?
I will eventually be hooking up a grid-connected, Net Metering system (tie in wind power or PV cells to an inverter... sell excess back to utility when I'm not using all I generate, buy some from utility when my needs outstrip what I'm generating locally). Even though the controllers on the inverters for these systems automatically disconnect if utility power goes down, the electric company still wants the same manual disconnects they require for someone with a backup generator. Hopefully, what ever I put in now will still be usable when the net metering system goes online.
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Features to look for in stand alone generator
Tom -
For more info on Net Metering (or Grid Interconnect systems as they are sometimes called) check out Home Power Magazine at the link I hope I attached below.
I would not be using my generator to feed back into the grid. That's too expensive. (Initially, it's for power outages from the grid and portable power needs.) What most Grid Interconnect systems do is synch the inverter (which takes the DC from you solar cells, windmill, or mini-hydro system) to the utility power. When you are generating more than you use, you run your meter backwards, and are selling power to the utility. When you need more than you generate, your meter runs forwards and you are buying from the utility (hence the term "Net Metering".
The disadvantage of this system is you still have to pay to get the utility power installed at your site (unlike a completely off-the-grid system). The advantage is that you don't have to buy and maintain a bank of batteries... the utility grid acts as your battery. Another advantage is that you don't have to size your "Green" power system to handle your whole load. Surges for starting up motors, for example come from the grid, if needed. You can also start small and slowly build up to a system that meets all or most of your needs, as opposed to an off-grid system where you have to buy a good sized system from the start.
A lot of state in the US have Net Metering laws which require utilities to buy back this "green power". Vermont is one of these, and from what I've heard, most of the utilities are OK to work with on it. (In some states, the utilities make it a real hassle, even though the law requires them to do it).
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02-01-2002, 13:27 
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