[mesquaukee]

Check out the do it yourself 12 volt systems such as Nova Kool at www.southerncalmarine.com/product_info.php/cPath/24_47/products_id/112 It’s about $900. It's the least expensive system I know of. I had a Nova Kool for 4 years in the tropics before I built a larger more efficient system.
This page gives you an idea about energy use www.boatelectric.com/novalt200-rt6%20&%20lt200.htm
There are a bunch of other do it yourself systems that are larger and more expensive.. All of them come pre-charged with quick disconnect fittings.
12 volt cold holdover plate systems are expensive and use considerably more energy than the above type of systems. The reason is very simple. The evaporator pressure has to be much lower to freeze the solution in the cold plates. This translates into longer run times for the same amount of heat moved.
Cold holdover plate systems which run off the engine are horribly expensive. They are great if you run your engine 2 or so hours a day for other reasons than cooling your fridge.

Have fun, Tom