Almost all of the current crop of DC generators are 3 phase alternators
with 6 big diodes. The alternator form is usually more efficient in
terms of heat losses and such plus the cost to build it is much less
than the incremental cost of the diodes.

Which brings one to the issue of genset efficiency: You want to
optimize electricity out for fuel in. I went looking for a bookmarked
set of fuel economy tests on small generators, couldn't find it. IIRC,
you need to be at about 2/3rds of the rated hp converted to the
equivalent kw output to get the best efficiency in the single cylinder
engines up to perhaps 10 hp.

I did find some other sites with outlandish comments. This fellow claims
that a small generator is better, then has a table that shows the kwh
per gallon of gas is much higher for the larger generators. Go figure!
http://www.endtimesreport.com/Generators.html

wrote:
Hey folks, I've been having trouble getting an answer to my question
from off grid sites. I'm hoping someone can help me or point to the
right place. I am planning an off grid system. The house will be run
off of a bank of batteries with an inverter to supply AC power. My
question is on the charging of the batteries. In general, is it
better to supply the batteries with DC power generated consistently
over time or to supply AC through a charge controller for shorter
periods. I plan to use solar and wind but may have to rely on a
generator for some periods. The generator is the part that has me
stumped. Tho the generator will run off renewable fuels, I'm unsure
whether to run a large generator making AC for a short time, or run a
smaller generator making DC power for a long time. I had considered
an engine small enough to drive generators the size of those found in
wind generators. Any help or direction would be appreciated. Thanks,
Paul