Older Frost Free Refigerators
I had one of these Kelvinator SXSs 20+ years ago in avocado, it had the WP built IM in the freezer section. It was a really well built refrigerator but I just didn't have a use for it and it did use a lot of power. I think the practical life span for FF refrigerators is about 30 years and maybe 40 if it is a really good one and it gets light use and good care. FF refrigerators will not last as long as a manual defrost machine and with the exception of the new energy efficient models will always use more power.
Paul I had a customer who had a late 60s Admiral SXS and it had at least 8 heaters in it, but the heaters usually are not the biggest problem with these early SXSs. The biggest problem we see is they run so hard and long and the compressors run so hot that they just do themselves in.
If you know what you are doing you can take many of these older refrigerators and reduce the power consumption in half, I have done this with most of the fridges that I have. The easy thing is to change the defrost timer so it only defrosts 1/2 as often and you can sometimes go far longer than that. 2nd thing we do is replace the condenser fan motor you can often go from 40 watts to as little as 6 watts by switching to one of the newer more reliable electronically controlled motors, you can do the same with the evaporator fan motors inside the freezer. This will usually result in a savings of at least 20 watts, but since this waste heat is inside the refrigerator you are saving almost 60 BTUs of heat the the compressor will not have to move. 3rd we look at the various heaters used inside and outside the refrigerator, some of these can be disconnected altogether others can be wired to only run with the compressor or a switch can be added so you can control them. Since smaller families, air conditioning, and living in the dry Southwest are more common now than when these fridges were built doing this in a smart way will not impact operation. And last we change out the older compressor to the newer capacitor run compressor and this alone can cut over a 100 watts of power draw.
You still will not get it to run as efficiently as a new SXS but it is a good compromise if you like the vintage machine. An older SXs like the Kelvinators pictured easily use four or more times the power of new models, some of these actually can cost $40 per month to operate.