"Larry" wrote Charging the battery with a 20 watt panel
and the tiniest fridge running
off the batteries will kill the battery 2 hours later than without the 20
watt panel....if the sun is bright, of course.

We need KILOWATT HOURS, not watt hours....Big difference!


Larry,

What you say is true if the boat is continually in use, like a liveaboard.
Lets say it puts out, on average 1amp for 8 hours on a nice sunny day in
Michigan. If the boat has a fridge, it probably draws about 2.5A on
average, so the panel will provide 40% of this for 1/3 of each day. Total
is 8 Ah/day or about 16 minutes motoring (on our boat).

But, if the boat sits at dock a good part of the time, the 20 Watt panel may
do some good. Lets say 6Ah/day for 5 days between weekend cruises. This
will put back 30Ah, or about 30% of what is needed to recharge a 200Ah bank
that is 50% discharged.

I have been thinking about installing one 80 or two 40-Watt units. These
would have capacity to recharge our 200Ah bank between cruises. When it is
operating at peak while cruising, it will put out the 5Amps that the fridge
draws when running. It will need a regulator and I would connect it to the
input of our battery combiner (which is actually the alternator output).

Still need to do some research on which unit(s) to buy.