A
Morgan 30 might do well with only 12,000 BTU unit. We use 16,000 now. You really are not going ti run A/C on 12 volts and be cool without a
generator. Even then you might still be better with AC power. 12,000 BTU's per hour requires better than 3500 watts of power assuming you had a perfect efficiency unit (nothing is). That would be about 3/4 of the contents in a 4 golf cart
battery bank per hour.
Wiring this up takes attention to detail where running the ductwork is mostly vexing. It's worth the
money to get a decent
pump for the
cooling water. March makes about the best and you can buy a spare
head for it should that fail.
Cal pumps are
cheap and tend to act that way and impossible to
service. You want the
salt water supply line as short as possible and it requires it's own sea strainer and through
hull. All the extras may or may not come with the AC unit but the quality of them matter a great deal. These door prizes add up to serious
money as does the labor to install them.
We have a Mermaid now and we had a
Cruisair before. Most all of them use a similar
compressor that is sealed and unserviceable. Both were reverse cycle and heated exceptionally well if the
water is above 50 degrees. You should look for units with a control
head that sets a thermostat. It makes the use just like at home.