Matt,
6hrs at 240watts or 8 hrs at 150 watts - either one would heat water by
about 100F. But what was the water temperature when you started?

If the engine had not been run, it might be say 80F, so we get 180F - not
enough to boil, but enough to scald anyone opening a faucet.

If the engine had been run, the batteries could very well be fully charged
and the hot water tank hot - lets say 150F. Now we anchor and put 240 watts
into the tank for the next 6 hrs. Now the water IS boiling!

Whether or not this is a real world problem, depends on how the boat is
used. For live-aboards or those on extended cruises, it is likely not an
issue. But, for weekend cruisers it could be.

Suppose we return from a long weekend cruise and the batteries are low. We
don't have shore power and rely on the solar panels to recharge the
batteries unattended during the week. We get 8hrs/day for 5 days of bright
sunshine on our 2x75watt array - about 430 Amp hours and we have a 200AH
bank plus starting battery that need a total of about 150AH to recharge.

In a case like the above will the charge controller safely cut off current
to the batteries? Or should we cover the array?

GBM

"Mark Borgerson" mborgerson.at.comcast.net
wrote
GBM wrote:

But, if the solar panels put out, say 150 watts for 8 hours, what
happens
when the water becomes too hot? With a small boat type water heater say
6 or
10 gal, this could happen quite quickly. Maybe then we throw the towel?

According to this handy calculator:

http://chuck-wright.com/calculators/watts.html

Heating 6 gallons of water by 100F will require about 240 Watts
for 6 hours. You're not likely to boil the water in in
the heater with that amount of input. With losses, 6 hours
at 150Watts is probably just enough for a hot shower. OTOH,
on a day sunny enough to give you 150 Watts for 6 hours,
you probably don't need a HOT shower. ;-)

Mark Borgerson