You are probably not using the battery to run a compressor for the total
cooling. It is probably a heat pump, with the Ocean as the heat sink. So
does not use all the electricity to get the BTU output.

"JAXAshby" wrote in message
...
comments interspersed.

don't understand your question, but will try to explain what _may_ have
been
your question.


I don't understand you explanation.....


An a/c unit will use about 1,400 watts electricity input to remove
every
1,000
watts of heat.

The Mermaid Marine a/c in question here is rated at 6500 BTU, and draws
6.4
amps at 120 volts. That is 768 watts of input power.

the 6,500 output is TOTAL output, which = input + heat removed.

Also, I used the figure the HVAC industry uses.


The a/c is rated at "6500 BTU", which is really 6500 BTU per hour
(manufactuers tend to leave off the "per hour" part of the
specification).
This unit has an EER (energy efficiency ratio) of 6500/768= 8.5, which is
comparable to a typical window mounted A/C.

see above.

.
6500 BTU/hour = 108 BTU/minute.
1 BTU/minute = 17.58 watts, so 108 BTU/minute * 17.58 = 1899 watts.

My calculations show that 768 watts of input power the unit will remove
1899
watts of heat. How did you get your number????

that's total removed heat, *including* input heat as well.


1,000 watts = 3,012 btu's(according to the figures used by th HVAC
industry [I
have family in the business long term])

1000 watts should be 56.89 BTU/minute, or 3413 BTU/hour.

so I hear, but the 3,012 (sometimes 3,014) is the number used by the HVAC
industry.

A group 27 battery usually has aboout 100 amp-hour capacity, of which
about 50%
is usuable. 2 grp 27's will give about a total of 100 amps before
going
dead,
as in unusable

100 amps at 12 volts = 1,200 watts = 3,600 btu's

Now you are mixing amp-hours and watts, not good!

amps used per hour times voltages IS watts per hour.


12 volts at a100 amps will yield 1200 watts, which will provide 4096
BTU/hour. Your estimate of the batteries being able to provide this
power
for an hour (fairly reasonable estimate) means that the batteries could
provide 4096 BTU of cooling.

see above to net heat removed, effective.


Also, a group 27 lead-acid battey will provide about 100 amp-hours. The
Optima batteries of the same size are considerably less.

across 1-1/2 hours that makes for about 2,400 btu's per hour INPUT
(about
800
watts, about 65 amps).

Which makes for about 1,800 btu's of cooling.

Lost me here. I calculate 4096 BTU of coolig.

total for 1-1/2 hours. reduce to 3,600 btu's for 1-1/2 hours, reduce for
HVAC
industry claimed effectiveness and you are down to 1,800 btu's per hour
heat
removed from the living area.

btw, 1,800 btu's is about the cooling capacity of 12# of ice melting.

Large airconditioning systems are often rated in "tons". A "ton" is
defined
here as the amount of energy required to melt a ton of ice. A ton is
2000
pounds, and requires 12,000 BTU to melt.

144 btu's per pound "change of state" (32* ice to 32* water) times 2,000
pounds
= 288,000 btu's.

"tons of ice" were used originally by a/c (and reefer) manufacturers to
give
(commercial) customers a handle to under the "how much cold" the units
could
deliver. the term is still used.


2000 lbs ice / 12000 BTU * 1800 BTU = 300 pounds of ice.

I think your math is a little off today.

1,800 btu's per hour/144 (btu's per pound of ice "change of state") = 12.5
pounds of ice times 1-1/2 hours = 18+ pounds of ice.

no way in hell will a marine a/c unit make 300 pounds of ice in a hour.


In other words, for drawing your 2 grp 27 batteries to their knees in
an
hour
and a half you will get about the cooling capacity of throwing about 18
pounds
of ice on your cabin sole.

Do you really believe that 18 pounds of ice sitting on the floor will
cool
the cabin as well as a 6500 BTU airconditioner????

of course.


The bottom line is that it will be very difficult to run the
airconditioner
off of battery power. If the cabin was really well insulated, and the
difference between your set point and the outside temperature was not too
great then you might get several hours of use from a bank of 4 Optima
batteries. Better have a separate battery for starting the engine
though.

nah. no vendor was able to stay in business trying to sell that to the
boating
public. Some people bought the story and the product, but all changed out
shortly. There simply was not enough battery capacity available to run
the
unit

btw, 1,800 btu's net ain't a hell of a lot of cooling capacity AND to
recharge
the batteries you need to produce about 90 amps PER HOUR for each hour run
by
the a/c. 12 hours run off the batteries requires 12 hours of engine run
time
at 90 amps charging. (that is figuring about 33% charging losses, though
some
will say 20% is obtainable)


Rod