Power consumption for 12 volt refrigeration.

Glenn, I have developed a base line theory for projecting daily
electrical power consumption when planning for 12 volt refrigeration
on a boat. You may remember the Cruising World magazine article some
twenty years ago that GB is still proud of, in this article my boat
box heat load estimate was higher than the rest. I have found that
heat energy forces on small refrigerated boxes can be more important
in some cases than adding insulation R value. There are three factors
influencing daily refrigerator power consumption beyond that of R
value, most creative marketing people fail to point out, cruising
climate conditions, refrigerated product through put and the true
System COP of units purchased. In the past the installer or owner had
no control over matching system COP after installation but today with
Danfoss BD variable speed compressors they can adjust a system's
efficiency.
I usually recommend insulation R value of 30 for boat freezers as
they are not operated at box temperatures below zero degrees F.
My theory on amperage required per day is based on worst case actual
and simulated conditions. The base line conditions I work with are 90
degrees ambient air at midnight, seawater temp 86 degrees and three
inches of new polyurethane insulation. I use a standard box heat load
requiring 10 amp-hrs for each cu ft of refrigerator and 20 amp-hrs for
each cu ft of freezer per day, this is based on top loading boxes and
two p3eople on board. Once the worst case base line is established for
a given size box then it is possible to use the only stable factor in
this equation water temperature to adjust for less than tropical
conditions. For each degree of cooler seawater the energy usage can be
reduced by 2%. If a boat is now operated in seawater temperature of
61 degrees refrigerator power consumption projection can be reduced by
50%. If is easy to see why forum reports from west coast boaters seem
to get better performance figures than boats on the South East coast.

Danfoss does not recommend running BD compressors direct from a power
supply they have indicated that a battery is necessary to dampen
voltage spikes. The electrical module is Danfoss's weakest link and
the new three pin units seem to fail as frequently as their six
predecessors. Module failures seem to be related to improper size boat
wiring and not connecting power source direct to battery or main power
buss.