I spent a long, hot summer as a student engineer working for Procter and Gamble in Kansas City, Kansas. The plant made Zest soap among other things. The tallow used to make the Zest came into the plant by rail in huge black tank cars. The tank cars had coils of pipe inside so one could hook up a steam hose to the heating coil, turn on the steam, wait a few hours, and quickly drain the warm, melted tallow out of the tank car. Filling and draining tank cars is rather messy, so there was always some tallow that splashed onto the cars and on the ground. Add days of breathless summertime heat and intense sun in the city's Missouri River bottom, and the tallow quickly went rancid, leaving me with an unmistakably, unforgettably pungent memory of those days.
And, no, to this day I cannot stand to use Zest.
Anyways, back to your question.
Look for a thermostatically controlled pail/bucket heater. There are heaters that are just a band that fastens around the pail and others that add a layer of insulation for faster heating and energy efficiency. Be sure to get one specifically rated for plastic (polyethylene) containers. The ones not rated for use with plastic can get hot enough to melt plastic -- not good for the bucket, nor for your oils. There are some heaters that are not thermostatically controlled -- they just have a high temperature safety shutoff -- and I don't think I'd want them for soaping fats, even though these heaters are cheaper. I also would not get an immersion heater -- a wand that you stick into the product. They are messy, hard to clean, and because they are mostly used for heating water, they can get very hot and have no thermostatic control.
I've been looking on the Webstaurant Store at soup kettles. There are 2 made by Avantco that hold 11 quarts that are reasonably priced at 60.00 and 80.00 with stainless steel inserts. Temp holding ranges are between 140-160 degrees. Standing by a microwave just doesn't appeal to me for MP. The only downside is having to use a ladle vs. having a pouring spout. Just a thought
DeeAnna, you are a genius. I have a Nesco (spelling?) roaster that I used to cater with for my husband's training classes. The footprint wouldn't take up more space than 2 of the soup kettles and the buffet inserts would be the best size for me (hobbyist). I can take that item off my shopping list!
I suppose the roaster could be used by putting the solid fat directly into the roaster pan. Or this might work better -- fill the roster pan partly full of water, set the pail of fat in the water, and turn the roaster to the desired temp -- you'd be using the roaster basically as a hot water bath (bain marie). Then you could pour right out of the pail into your soap pot, rather than deal with ladling from a roaster pan of warm fat. (And having yet another thing to wash!)
"...When OP talked about oil tank, I thought she meant one of these..."
You could certainly be right, Lee. You're looking dedicated tank with a permanent heating system on the outside. A water jacket can provide a more gentle heat, compared with a band-type heater. The issue is that you need to get the fats/oils decanted into the tank. If you're looking at coconut oil, for example, in a 5 gallon pail, you'd probably want to liquefy the CO in the pail so you could pour it into the tank. So a band heater might still be useful.
"...a 30/40 quart stock pot and an oven (if it's tall enough)..."
Warm/hot air is surprisingly inefficient at heating a large solid mass, especially if you don't have forced air movement. If you're looking at energy efficiency, an insulated direct-type heater such as the water jacket or band heater will do the job faster and more efficiently.
"...Do you think the roaster pan is big enough and sturdy enough to sit a 50lb pail of fat into it? ..."
I don't know -- never tried it. But the conversation has morphed a bit into what one can do to warm smaller jugs and pails, and a roaster pan should be sturdy enough for that and should be large enough to hold a couple of smaller jugs/pails.
Thanks everyone for the suggestions. I was originally thinking about the heating tank with spigot. I have started doing more melt and pour and since I have a separate soaping room now with no stove, I thought a tank would be handy to melt. Microwave is a PITA for larger batches.
I also thought it would be handy for larger masterbatches of CP. I don't want to spend $1000 right now so some of your other creative ideas are interesting. I will look into the belt heater and that that roaster looks intriguing. But I really like the idea of pouring from a spigot. hmmmm....