question about efficiency of generators

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have a (new) 90 amp dls-90 iowta battery charger, drawing 1500 watts;
occasionally, will need to charge a 2000 amp hr battery bank at 12 volts;

someone gave me an old 1800 watt 3.5 hp craftsman gas generator,
which took a few hours to fix; also have a 5500 watt no-name
gas generator;

which generator is expected to be the most efficient at producing the
electricity, the 1800 watt running at about 80% capacity, or the
5500 watt running at about 25% capacity?

am thinking of putting 1/2 gal of gas in each, and making an attempt
to measure input, but my "cheap" meters loose all information when
power is cut; any suggestion on how to accurately measure the input?

thanks for any suggestions

Paul

 

[William P.N. Smith]

which generator is expected to be the most efficient at producing the
electricity, the 1800 watt running at about 80% capacity, or the
5500 watt running at about 25% capacity?

No question, the 1800W running at 80%. Check the recent discussion in
alt.energy.homepower about generator fuel consumption.