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# Generator efficiency

## Generator efficiency

#1
08-17-12, 12:45 PM
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Join Date: Aug 2012
Posts: 189
Generator efficiency

My 7 kW Generac LP powered generator has a 14.5 horse power engine.

Using the formula 1 hp = 0.7457 kW would mean it requires 9.4 hp if the generator was 100% efficient. So does this mean my generator runs at 65% efficiency?

How do these portable generators get ratings up to 8 kW?

Last edited by DMCman; 08-17-12 at 01:10 PM.
#2
08-17-12, 01:46 PM
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Join Date: Mar 2006
Location: Wet side of Washington state.
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It is a combination of efficiencies but you are correct that the end result from a purely mathematical perspective is that you only get a fraction of the heating content of the fuel used back as electrical power.

Almost any generator will have peak outputs exceeding their continuous load outputs. It is a matter of heat, mostly getting rid of excess heat, that limits the peak output to a short period of time. There is also the peak output of the prime mover (engine) to consider.

#3
08-18-12, 04:07 AM
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that you only get a fraction of the heating content of the fuel used back as electrical power.

Which could explain why the generators had a higher capacity using LP vs. natural gas?

For example, can't remember if they still do but, the 20 kw was rated for 18 on natural and 20 on LP. It was like 7/8, 13/14, 16/17, 18/20 natural/LP

#4
08-18-12, 05:19 AM
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Join Date: Aug 2012
Posts: 189
That is correct that NG has less horse power than LP on the model type I have.

But I was just looking for the typical efficiency of the electrical generator, not the engine efficiency.

Do some have better designs (maybe permanent magnet type) then the one I have which has a two pole rotor and stator?

Last edited by ray2047; 08-18-12 at 07:15 AM. Reason: Remove special font.
#5
08-18-12, 07:16 AM
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DMCman, Please do not make your posts harder to read by using a font that displays smaller then default. There is usually no reason to alter font or text color.

#6
08-18-12, 09:24 AM
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Join Date: Aug 2012
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Sorry about the font. I usally run a spell check with a copy and paste and that software much have changed the front.

#7
08-18-12, 09:40 AM
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Both Opera and Firefox have easy to see in line spell check built in.

#8
08-18-12, 12:25 PM
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Join Date: Feb 2002
Location: port chester n y
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The best method of determining the performance of you generator is to calculate the amount of gas , measured in cubic-ft , that is consumed when the generator produces one Kilo-Wattt Hour of electricity.

You could connect an exact load of 1000 Watts to the generator and run the generator for exactly one hour, but you would need a gas-flow to measure the volume of gas consumed during this one-hour period.

Possibly there are "gas-experts" in other sections of this DIY Forum who could advise you on the practicality of connecting a gas-flow meter to your system.