Just to add some clarification to Thumbs Up's comments so a new round of Chat room myth isn't created regarding the variable speed of the Danfoss compressor. You know that saying, "a little knowledge is dangerous", so lets flush out this topic a little more. And besides I don't really want to start my final paint
coat on the forward head
yet...so this is called Boat Project
For a Rolled Aluminum Evaporator:
The reason variable speed is nice and can save power is that it does you no good at all to run your compressor at a faster speed than your evaporator can remove Heat from the box. The evaporator surface area needs to be matched to the box heat load and compressor running speed. So if you run your compressor too fast...well you are spending Amps that don't remove heat...equals inefficiency. So on thin rolled aluminum
evaporators, a variable speed controller WILL save you power at the end of the day. Sure you can swap out and play with resisters manually but at 3PM you may need one compressor speed when your boat temp is 90-degs F but at 4AM when it is at 80-degs you need another speed due to the different amounts of heat that are entering the box. Heat is only removed from the box when the compressor is running! So doing it manually just isn't going to give you the benefits as paying the little bit extra for a variable speed controller. They also make lots of after market add-ons to make your single
speed controller into a variable speed.
For a Holding Plate type Evaporator:
This type of system is designed around the compressor being operated at a speed or we can say "heat pull out speed" faster than what the evaporator (holding plate) can pull out of the box. The point of this design is to freeze down the temperature of the eutectic liquid inside the plate in addition to pulling out heat. Holding plate systems continue to remove heat when the compressor is off, so you need/want to store up "cold" in the plate. So on holding plate system, you don't gain anything efficiency wise by varying the speed of the compressor to match your box heat load. You just set the compressor at the most efficient speed and you want it to be "mismatched"...or over-sized...for the real time heat entering the box. This is how you get the longer off cycles with a holding plate set-up.
What you use the speed control for on a holding plate system is control the length of the compressor on cycle, in other words how long it will take to freeze down the holding plate. While this will control your instantaneous amp usage the end of the day number will be about the same! For example if you use 2.5A at low speed and it takes 2hours to freeze down the holding plate that 5AH. Or if you run the compressor on high speed and it takes 5A but only needs to run for 1hour...bingo...you will use the same 5AH per day. Personally, I would rather go for the faster holding plate pull down and use more real time amps, that way when I load the box or put in a big Dorado it freezes faster.
I get this question all the time from folks wanting to add a smart speed to their holding plate system because their "dock buddy expert" told them it will save them 37.5% on their power usage, because he read it online...
...but not so fast. While automatic speed control is a good thing on a evaporation system, manual speed control is the best for a holding plate systems, set it and forget it.
Speaking of a Big Dorado....
45lb monster caught in the Cervalo channel sailing into Pa Paz.
It took me 45 minutes to get aboard and we had to douse sails
and reverse the engine
down on this guy...so yes that is sweat on my shirt...oh the Memories.