I would recommend against using a DC supply. A simple transformer (120V to 24V) is ideal and gives AC power.
The problem with DC is that it is more likely to support corrosion. AC is typically used for water sensing and other wet applications to avoid corrosion or dendritic growth.
I used a transformer rated at 100W on the secondary, but this was more than needed. I would go with something with some additional margin, a little more than 25W. (at 24V, the resistor will cause a 1A current draw or about 24W) You don't want to stress the transformer with a 100% load, it will probably overheat and shorten its life.
I think that this topic has come up in the past on WxForum. Search around and you should find what others have done including
my own installation.
I bought a cheap temperature controller with an external sensor. I have it apply power only when the ambient temp approaches freezing.
Greg H.
p.s. added a photo of my transformer (eBay) and controller.