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
As the load is decreased, so is the fuel consumption. It's like keeping
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
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
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
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.
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.
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.
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
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.
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
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.
We're *all* in agreement here-
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
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.]
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.
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?
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
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
Actually, in actual use a 3500 run at full tilt MAY use as much or
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.
The Honda gensets referred to are INVERTER generators - they are
variable speed units, generating DC which is converted by an inverter
to AC and speed varies directly with load - which means a LOT less
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