Oversized generator

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Hi,
If I buy a generator that's too big for average use for my house - for example, I really need a 3500 but get a 7000 instead - and most of the time don't draw more than 3500, will I be wasting have the gas or are the generators smart enough these days to burn only as much gas as needed?
Thanks,
Sam
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On 10/6/2011 7:54 PM, Sam Takoy wrote:

a car at 30 MPH, on a flat level road, it requires less gas than when it's going up hill, to maintain the same speed. Typically, the generator spec will give you fuel consumption at full load, and half load
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On Sat, 08 Oct 2011 07:52:57 -0400, Stormin Mormon wrote:

A real Mini Cooper, or one of those modern BMW pieces of sh*t? ;-)
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Sam Takoy wrote:

If cost is not a big concern, I'd always take over sized one than just right or undersized one in anything.
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On 10/6/2011 8:42 PM, Tony Hwang wrote:

My Generac 7KW was the smallest standby generator sold in 2006, and I bought it since it had more than enough power to carry my basic home needs (furnace, refrigerator, sump pump, most lighting, computers, TVs, garage door opener, microwave, etc.)
The idling consumption of gas is about 50% lower than the idling consumption on the next larger model, and thus a partially loaded 7KW generator consumes considerably less fuel than the same load on a 10 or 15KW unit. This results in a simpler installer (smaller diameter gas line and no need to upgrade the gas meter) as well as lower operating costs as well as lower first installation cost by at least $1500 saved up front.
The 7KW generator is smaller, quieter, and cheaper, and made the most sense to me. I have never regretted using a minimalistic approach here.
To be fair, the negative side is that I cannot supply enough power to run my home central a/c, my electric ovens, or my electric clothes dryer. And I suppose that my single cylinder engine will not last as long as the 2 cylinder engines used in the more expensive models.
Generac, Kohler, and others have deliberate;y made the transfer switch / breaker panel for the cheaper models offer fewer circuits. This limits the number of home devices which can be powered, and some people buy the larger generators so as to have more powered circuits supported. Careful rearrangement of branch circuits in your home can overcome some of this constraint.
I therefore would vote for having LESS rather than more in a standby generator, and would certainly buy another smaller unit like the 7KW if I was doing it over again.
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It will also last longer not needing to work as hard
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It will be reduced, but not by half. Even with no load, an engine of a given size must burn a certain amount of fuel just to stay running. You can also run some cable to your neighbor and have him chip in for gas.
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On 10/6/2011 7:54 PM, Sam Takoy wrote:

As far as I know, all are specified as running wattage and peak wattage. There will be times that a couple of things start at once needing the peak.
My neighbor's generator is half the size of mine and he gets by but wishes it was larger.
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In most cases it will burn less if it is working less. There is some "overhead" though. But in the less than 10k watts it's not a lot.
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It depends on your generator. Mine is a 5500 watt & does not change a whit when it is running with no load, or a full load.
My neighbor, who works on Honda's, says Honda's have the best 'speed regulating' setup.
But, as others have pointed out; 1. you won't cut consumption in 1/2. 2. too big is generally better than too small.
I've never heard anyone complain about their generator being too big. It is not a very efficient beast to begin with. It is for comfort-- more comfort is good.
Jim
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That's hard to imagine. I've yet to see a generator that does not have a governor that regulates the throttle based on load. Let's assume you had gasoline flowing at a rate to support 5500 watts, but there is no load connected at all. Where would all that extra energy go to? If it went to waste heat, it would generate one hell of a lot of heat. Whether it's a lawn mower, car or generator, all regulate gas flow to the load.
To the OP, that generator will use a lot less at half load than at full load. Think of two identical cars except for different engines going 100mph. One car has a 2L V6, the other a 5L V8. The 5L is going to use somewhat more gas, but while the 2L is maxed out at 100mph, the other car can go a lot faster if needed, burning more gas to do it.
In the grand scheme of things, if the generator is for occasional use, I'd much rather have the larger one because the additional gas usage cost isn't that great and having the extra capacity is almost always a good thing.

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wrote:

I believe what he is saying is that no matter what the load the generator is always running at the same rpm.
The governor increases the throttle opening when the generator comes under a load to keep it at the same rpm and output frequency. For most gas ones that's 3600 rpm. Much bigger ones will run at 1800 rpm.
To a point the gas consumption is not much more. But you can get carried away. A 20kw generator will be a multiple cylinder engine and will burn a bit of fuel even with a small load. Just to keep the engine turning at 1800 rpm.
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He never mentioned RPM. He only talked about how much gas it uses versus load.

Agree.
Agree.
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-snip-
-snip-
We're *all* in agreement here-
-snip-

There is some relationship there, I thought. I've never paid a whole lot of attention to gas consumption. Mine holds 7 gallons and says it will get up to 13 hours from a tankful. That has been roughly my experience whether I have been running it with 1/2 my house hooked to it-- or doing the monthly workout with a power tool or heater hooked to it. There may well be a bit of difference in consumption--- but it isn't enough so that I notice it.
The RPMs are a constant, though they might dip when a big load is first applied.

We all agree in principal. The only quibble seems to be *how* much more some generators use while under [*how* much?] load vs. others.
My neighbor, who works on them, says [some?] Hondas are very efficient when used at less than max load. That might be just paper theory but if the OP cares, it is worth looking into. [Neighbor has been a small engine mechanic for 40 years. He doesn't own or sell generators- but in general he knows his stuff.]
Jim
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The simple home generators must turn at 3600 RPM all the time to put out the 120/240 60 hz voltage. It takes so much gas to do this. Then as the load goes up, so does the fuel usage. Some Honda generators use inverters. This allows them to cut the speed back under no or small loads. This will reduce the ammount of fuel used when the load is small. When the load goes up, so does the speed of the motor.
Where the simple gasoline generators just depend on the 3600 rpm of the engine, the ones with invertes depend on the electronics to keep the voltage/frequency constant. The engine can run at any speed that is needed. While I have not checked it out, it may be that the ones without the inverters can handle large surge currrents such as starting motors beter.The inverter will shut down,but the direct drives will try to put out the surge current for a second or two.
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On Fri, 7 Oct 2011 11:32:51 -0400, "Ralph Mowery"

Thanks- Do you know how much difference that makes in actual use? For instance- to use the op's numbers; if you had a 7000W inverter type running 3500W -- vs a 3500W non-inverter type- how much fuel would each burn?
Jim
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It would take decades to recover the price difference. I doubt you can find an inverter style 3kw generator for under 2 grand. I paid $500 for my generic 4.4kw generator.
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On 10/7/2011 11:17 AM, Jim Elbrecht wrote: ...

Well, here's a (useless since it has no scales; you can tell it's a marketing tool) graph from Honda showing they're just the cat's meow in that regard.
Try a ping to Honda referring to the graph and ask for the supporting data and see how far you get... :)
Undoubtedly since there's less load (torque applied) it requires less input power. How much that translates into fuel saving versus load is anybody's guess.
--
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wrote:

more fuel as a 7000 run at half load. GENERALLY the specific fuel consumption is a bit better when not run at full load, because even relatively simple small engine carbs can have full load enrichment circuits that richen the mixture at full power. Will the differnce be large? No - in either direction. I'd opt for the 7K unit - particularly if it is an 1800 rpm unit instead of a 3600. Not too many 1800 rpm units under 5500 watts - and they get more common above 8000 from what I remember.
I DO have a 3500 watt 1800 rpm Onan. It's a "senior citizen" with a rope start (not recoil) only - so that gives you an idea of it's age.
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wrote:

variable speed units, generating DC which is converted by an inverter to AC and speed varies directly with load - which means a LOT less "overhead"
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