On Tue, 15 Sep 2009 15:52:35 -0700 (PDT), "
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

Generally, you size your renewable supply (solar or wind or whatever)
to keep up with your consumption, plus some reserve charging capacity
to get recharged in a reasonable period after discharging your
batteries. Then you size the backup generator to charge at a similar
rate. For example, at my place www.citlink.net/~wmbjk/solar100.htm,
solar is 2000W nameplate, and wind is 1000W. Charge rates vary
considerably depending on whether one or the other or both sources are
generating, and how much load is concurrent. There's really no such
thing as average, but I'd say that the normal charging rate we see is
about 40A after loads. Our primary backup generator produces about
70A. We rarely discharge below about 70%, and that generator can get
us back up to about 90% in 3 or 4 hours, depending on loads. For a
while I used a 140A generator, but that was too much. You'd need to
supply a lot of information to get a good recommendation on the best
balance for your application, and even then opinions will vary. But in
general, you want the *smallest* generator that will do the job. I
suggest that you post your info at alt.energy.homepower, or at the
Outback forum. Either way, lots of knowledgeable folks will offer
real-world experience.

Wayne