Efficiency of 5 to 10 KW Gasoline Generators for Residential Use

Thermal efficiency of a spark ignition engine is approximately 25%, conversion efficiency for the alternator perhaps 80%, that leaves an overall efficiency of perhaps 20% which is probably optimistic particularly at partial load. therefore a gallon of gasoline may yield about 7.6 KWH, therefor considering a gallon of gasoline costs between $2.50 to $3.00 a KWH will cost between $.38 to $.39 per KWH for fuel only, and that's optimistic.

I have used small gasoline powered generators on work sites, and estimate the efficiency may be realistically half that. then you must consider lube oil changes and maintenance,

Small gasoline powered Generators are convenient for work site, remote, and emergency applications, but can't compete with utility power.

Regards CEKM

Don't ask me how much I paid for a full tank for my pick-up truck next day after Ike hurricane.

GA Milo. Had my NH home outfitted with a 8KW gas back up generator (heating is gas) with auto Gen-Tran switch panel, so winter heating would be maintained while I'm not there during an ice storm (like last year). Good to know it is more efficient than gasoline, but you are correct.........based on the laws of supply and demand, the cost per KWH (+damage saved) is incalculable!

Nice. I didn't do the auto switch, Mine is manual- "Arm-strong"

I put in a separate service box with the lines I wanted to feed - direct to my gas boiler, my kitchen, and a couple of lighting circuits/outlets on each floor. Its right beside my main panel. in the garage. I made a high amp jumper cable to run from the genset toa high amp plug that feeds the secondary panel.

In case of extended power emergency:

1) switch off main panel from mains,

2) hook up generator

3) turn on generator

4) Throw main switch on top of 2dary panel.

WHen I run to the secondary box from the generator, I lock the main panel door shut so noone can accidentally back feed the grid by just "seeing if the power is back on"

5 days with no utilities in Georgia with small kids and roads closed made me a quick learner.

milo

There are small diesel gen sets out there, but they tend to be costly. See Northern Equipment on the web/net. Another alternative would be a PTO driven generator you can run off your tractor. These can be more efficient than a cheap gas drive as you can generally get 2 to 3 times the work from a gallon of diesel than a gallon of gas.

I always think in amps. To me 5,000 watts is 50 amps.

Hair dryer takes ten amps. Coffee maker takes ten amps. Saw takes 13 amps. The refrigerator takes ten amps... That's the way I think about in 110, 120 volts.

I wish I had a generator for emergencies.

We get Ice Storms around here where I live, which I have now been called to keep secret.

A cool thing to do is run out of the generator to the 220 plug that runs either your dryer, or your electric stove a line that puts out of the generator, 110 into the house.

I've lost my drawing of how you drop the leg in the 220 plug so as to be able to feed the house off the generator 110 from one line into the house.

God I'm getting rusty!

Still I do know it is not that hard to do, and could probably do it if I had a generator and just wanted to run one line into the house, and leave the generator outside.

It is important to cut the bullswitch off and isolate the house from the little gen feed, since otherwise the grid will demand that the little generator feed it, and the generator will not be able to, and will blow up.

More importantly, you must disconnect completely from the grid before running the gen set in an emergency to avoid KILLING THE LINEMEN that are repairing the line. Electricity transforms up as well as down. CJM

Gasoline is not a great fuel for a generator because you are paying road tax and because the engine in which it runs is not particularly efficient. A small, air-cooled diesel is about 32% efficient (and can run on natural gas). Using 80% for the generator gives a net of 26%, so for one therm of natural gas, you'd get 26,000 btu equivalent output: 7.6 kWh. Around here, a therm is $1.00, so the cost per kilowatt hour would be 13 cents -- more than our local electricity cost, but not too bad.

what about this use?

At these prices I should disconnect PoCo. I guess the real cost of ownership for these small generators is in maintenance and repairs and putting up with the noise. With Economy of Scale, I'd think commercial prices for elec. generated at 99% efficiency would be 4x-5x lower than these numbers.

I doubt that fueled commercial power plants exceed 40% efficiency. The only way I think you're going to come out ahead is if you happen to live in a cold climate, and use a diesel , LPG or natural gas generator.

Build a heat exchanger and heat your house, most of the energy is going out in the exhaust, get a liquid cooled unit and salvage even more.

Of course this leaves you with oil change intervals recommended between 50 and 250 hours depending on the quality of the unit so maintenance costs and your time would have to be considered.

Not to mention the life expectancy of the generator and replacement cost.

The average generation efficiency, from well to plug, is 38% in the US. Many coal-fired plants are only 33% efficient. Recent natural gas cogeneration turbines are about 60% efficient.

Hi All!

I came across a gen set that I never seen or heard of before. It was awesome! you know how most of them are screaming at like 3600 rpm or whatever a convient multiple of 60 cy would be and use a alternator to make the power. The crazy thing screaming whether it's under full load or drawing 1 amp..

but this one I seen the other day was at a worksite it was a DC gen set and like a converter from a UPS after it that made the 110vac output.

the rpm at no load was at idle, and as you drew more current it sped up to supply the power needed. soo cool, I didn't get a name of it but way cool,,

Now something I always thought of doing and even more so with this new generator, is during the colder months, us it to generate the power we need for the house, but also try to fins a water cooled engine if possible, and if not that at least run the exhaust through a air to water heat exchanger and store all that tons of heat to heat the house, or any other hot water needs.

I wonder what the efficiency then works out to be? if you reclaim most of the waste heat to help heat the home as well as generate power.

thoughts?

The generator you saw was one of the inverter units (from Honda or Yamaha, probably -- although others are making them now, I would guess.) They are quite cool, and easy to parallel if you need twice the output.

Your cogeneration idea would work well. Honda also makes such a unit, and they claim 85% overall efficiency, which is far better than the power company does, even with their cogeneration units. If you used natural gas as the fuel (which is how the Honda is designed) it would be very economical, efficient, and clean. The efficiency only "works" if you have a need for the hot water, which would virtually always be the sace in the winter (you'd use it for house heating). In the summer, you might produce more domestic hot water than you could use, if all of you electrical needs were being fed by the generator. Probably best to size the unit for summer, and make use natural gas for heating in the winter.

Hello Blink:

I Didn't Know That Honda Was Making a Cogen Unit. I Was Unable to Find Anything but the Press Releases Online for a 1 KW Electrical Unit Which Generated 3 KW of Heat.

Now If You Really Wanted to Get Tricky, There Has Been Renewed Interest in Absorption Units. I Was in a printing plant Where They had Installed a Chinese Made (but It Seems Everything Is Made in China) Absorption Chiller That Would Operate with 190° Water.

They Were Using Three Generators in the Neighborhood of 80 KW Each to Generate the Plant's Electricity, Selling Any Surplus Power Back to Edison, Cooling Heating and Dehumidifier the Plant Utilizing the Heat Scavenged from the Cooling Systems and Exhaust by Heat Exchanger's.

Unfortunately the System Never Worked Correctly. Maybe Someday Someone Will Follow Hondas Lead an Engineer Such a System from the Ground up. Talk about Green and Energy-Efficient.

Maintenance Would Still Seem to Be an Ongoing Problem for Homeowner Type Systems Utilizing Internal Combustion Engines. Do You Think Any of That Newfangled Synthetic Oil Would Significantly Extend the Maintenance Intervals?

I visited a production machine shop that had a couple of big air compressors. they had them off to the side in a little 'alcove' or 'vestibule.'

But the cool neat thing was, they had the waste heat ducted two ways- outsde in the summer; Inside into the shop in the winter. Just diverted the air to get rid of or recover the heat.

milo

Hello Milo:

I love it, low-tech energy efficient and cost-effective. Years ago a engineer in a shop where I was working as a apprentice (many many years ago!) Refitted his air-conditioning condenser with a water cooled condenser coil. He used his swimming pool as a cooling tower.

It had the desired results, the amp draw on the compressor dropped, the circulating pump use less energy than the condenser fan, and the pool was a little more comfortable, he claimed it warmed up by about three or four degrees.

As far as generators they certainly can come in handy, prolonged power outages are a rarity here in Southern California, but they do occur.

A few years ago we lost power at the shop where I was working for about eight hours. Of course the company had just installed a voicemail system, I'm sure they did it as a status symbol you hardly need voicemail when there's only three people in the office, so the loss of power pretty much closed them down.

Fortunately there was a generator in the shop, a quick solution was to make up an extension cord with two male ends, shut off the main and branch circuit breakers and backfeed the circuit with the phone systems and computers .

5 days with no utilities in Georgia with small kids and roads closed made me a quick learner.

I don't know how you did it. If I were you I'd go down to your local pep boys and buy one of those cheap Chinese made generators as a backup in your situation you can't have too much redundancy.

I have a 5.5kW single cylinder Lombardini Diesel generator. Which I've lent to a mate who is off grid in the country, while he gets his 12.5kW repaired.

Despite being a rope start model it will start at -5 degrees Celsius with 1 foot of snow on the ground, first pull, and at 75% load will burn approximately 1.2 litres of diesel an hour. He is impressed by its overall performance in that its power output is rock steady 240Vac @50Hz (Australian standard mains power) irrespective of the demand. He calls it "My Headbanger" because you know when its running, he also reckons it was the best A$200 I've ever spent (I bought it second hand).

As for efficiency, well when you can rely on it to start and go first time every time, and run till the 15 litre tank is empty some 10 hours later. Without trashing any electronic equipment in the process. Its not bad, depending on what you pay for diesel (about A$1.30 a litre), note this motor could also run on WVO or B10 with a little tweaking.

Rule of thumb for small gensets is that Petrol powered are fine for short duration fluctuating loads, where as diesel ones prefer to run well loaded and for reasonable periods of time. My 5.5kW is good for emergency backup but you need at least 12kW to properly run a house. 2 or 3 kW is good for running power tools on work sites.

If you want to buy a new small genset, be aware that you'll pay for what you get. My mate(same one) bought 3 chinese made gensets(2 petrol, 1 diesel) none of them were any good (poor speed regulation, no altitude compensation and fell to bits while running). That's to be expected from a country that copies technology with out understanding it. Which is why he borrowed mine...

Typically Hondas are good, and Generic's are good, and Colman's are second rate, at least in the US.

As Milo said earlier, efficiency is not really what you are typically after, though of course it is nice if you get it.

Ever' wunst-in-awhile, editing is called for...

Of course you meant: "Generac", NOT "Generic"...

Having rewound a Generac (once), back-in-the-day (while working in a large electrical apparatus service shop), I can attest that the older ones, anyway, were well worth the investment...!

Best regards ~

Yes, I misspelled Generac.

By the way the reason I am told it is best to avoid Colmans has to do with plastic fuel tanks, and Briggs and Stratton engines.

I've little experience with Onans but, hear they are competitive.

Often for my portable generator use, crystal sync was required.

Here in California Coleman has had to switch to a Subaru engine to comply with small engine regs. It seems to be a pretty good emergency generator : 5kVA for $600. For regular use though, you can't beat the EU (inverter) series Honda's. I've used them a lot from 1kVA to 6.5kVA on job sites. And, unless you're trying to heat water or cook a turkey, 1-2kVA will cover most of a households needs. I'm not sure of the efficiency, but the 2kVA would run all work day on 2 gallons (US) of gas.

Hahahaha...first thing that went through my mind, as well. Sure don't want to be this guy with HIS "Generic Generator"

...and I love the brilliant advice given in response. Way to "restate the problem." too bad all sites aren't CR4...but shhh, don't let in too much riff-raff... ;)

Those cheap Chinese generators are nothing but grief. They have poor engine speed regulation, no altitude compensation, which is needed if your operating above sea level. They barely conform to any electrical standards, and often will just fall apart as they are running.

As for reliability? Yes they are very reliable, you can rely on them to let you down exactly when you need them the most. When (not if) they do break, just try to repair it. There is no service information or parts available. They do of course come with Chinese warranty, once its out of China its out of warranty...

Which is why Honda,Yamaha, Yanma & Hatz generators are so much better. Its not the initial price that gets you, its what it costs you when it fails.

Failure is always costly.