12 volt verses 120 volt?

[VicB]

Would love to hear y’all’s thoughts. I have an older Norcold 120/12volt refrigerator. Would it be better to buy another 120/12volt fridge OR buy a much more efficient 120 volt refrigerator and an inverter. I’ve heard they use up your battery, but at anchor for the day, my current one runs off battery and doesn’t do that great of a job. I’m thinking the 120v one do a much better job and wouldn’t use as much power, but I wanted to see if anybody in the group has experience doing that?

[km1125]

It does depend on the fridge, but a 120V one is not necessarily that much more efficient.

Depending on which model you have, the Norcolds (especially the older ones) actually operate on A.C. whether you provide them with an A.C. source or D.C. source. In some of their power supplies, the A.C. is just converted from 120VAC to 30VAC for the compressor and the D.C. is run through an inverter and converted also to 30VAC. In later models, they converted the 120VAC to 24VDC and the 12VDC to 24VDC, then had one inverter that converted that to 30VAC. The most recent models mostly use a DC variable-speed motor and a small computer to convert either the 120VAC or 12-24VDC to about 12VDC that is fed into a circuit that controls the speed of the compressor.

It sometimes seems a "120VAC fridge" is more efficient or runs better but a lot of that is because it's in an air-conditioned house, not sitting in a boat with the sun beating on it.

Now that all that's been said, you can certainly run a 120VAC fridge on an inverter so you can run it while underway or on batteries. You need to 'upsize' the inverter so it'll handle any surge of starting the compressor but when running it will probably not draw that much more than your Norcold. (most Norcolds use smaller compressors than a "regular" 120V fridge). Not sure what fridge you have in mind but you can take the running specs for it and add about 20% to get a good idea of how much power you'd be drawing from the batteries. If, for example, it's rated at 1 Amp at 120VAC, that'd be about 12 amps running on battery (1*120=120watts divided by 12 volts=10amps then 10*1.2=12Amps). Depending on how hot it is, the fridge might only run 50% of the time (same with Norcold)

One thing you'd really want if you were running an inverter like that would be a settable low-voltage cutoff. Many inverters have this built in, but they're usually too low.... like 10.5 volts. You'd want it to cut off at a much higher voltage...perhaps 11.5 so you don't damage the battery.

[Midnightsun]

Keep in mind an inverter when at idle still consumes quite a bit of juice. Something that needs to be checked in the spec before deciding on an inverter as some do much better than others. Some may draw 2 amps doing nothing which is almost equivalent to a fridge which draws about 4 amps when running so let's say 2a averaged out. There are some inverters out there that can consume as little at .3 amps at idle but you are going to pay for those. Then there is the efficiency when inverting which varies from brand to brand one needs to look at also.

[Georgeviking]

We use a 200 watt inverter to power a 3.3 cu ft fridge. It’s about 34” tall and use two 29 series 100 amp hour deep cycle batteries. This fridge I don’t think is on the scale of energy efficient it will run it about 48 hours at a clip. We also put a small freezer on which seems to eat up power as have lost battery at day and a half. Think my draw while testing was 6 amp 120 volt drawing 23 amp 12 volt. With the cost of two batteries inverter wiring etc I’m wondering if 12 volt on its own battery would be better due to lose of power in the converting stage etc….we re did the galley for more space and smaller fridge was easy to get 120 volt.