Once upon a time, a long time ago, there was only one brand of refrigerator that would work on 12v DC boat power. There must have been many thousands of them out there back in the day, predominately on powerboats, and a lot still exist to this day.
These refrigerators have one or two compressors typically mounted on the rear of the cabinet, and these compressors require 20v-30v AC for their operation. When 115v AC mains power is available, either from shore power or a generator, the power for the compressor(s) is supplied by using a simple transformer to lower the 115v AC down to the lower AC voltage required by the compressor(s).
But when the boat unplugs from the dock or the generator quits, the compressor is then powered from a built-in inverter that produces the required low voltage AC from the 12v DC supply. Due to losses inherent in inverters and the desire not to involve electronics unless absolutely necessary, these fridges would always be supplied with separate wiring for connection to both 115v AC and 12v DC, with the system automatically switching to 115v AC if it is available.
Fast forward to modern times, and we are now blessed to have marine fridges utilizing the super-efficient Secop (formally Danfoss) compressors that are powered by 12v or 24v DC. Under normal circumstances, when the vessel is at the dock and plugged in to shore power, the battery charger will be on and supplying the DC power for the fridge. The batteries are not involved in this process, as the DC from the charger does not go into and out of the batteries, but is supplied directly to the fridge from the charger.
In this situation there will always be 12v or 24v DC available, but if circumstances dictate that the charger will not be left on, or that at times there will be no 12v or 24v DC available,
then the standard DC-only controller on the compressor can be replaced by an AC/DC Controller which has inputs for both 12/24v DC and 115/230v AC. This has automatic power selection, using 115v/230v AC when available and switching automatically to 12v or 24v DC when the AC supply is interrupted. This AC/DC Controller “makes” its own 12v DC to operate the fan (or pump if water cooled), and there is a separate 12v DC output to power accessories such as lights, digital thermostats, etc., that need to be powered continuously.
So, if you are replacing one of those old-style fridges which is using both a 12v DC and 115v AC supply, don’t feel compelled to use both power sources just because they exist already. There are no efficiency gains to be had from using the 115v AC supply, and the AC/DC Controllers are more expensive to replace if/when damaged by water, lightning, etc.
But if you envisage a situation where at times there may not be a 12v or 24v DC supply available, then the AC/DC Controller option is worth considering.
Especially if what you need is on sale, at a great price – then yes, you should seize the day! This boating season ensure you have cold drinks and ample food on board; no more lugging bags of ice or cursing yet another refrigeration malfunction.