A 250w heat bulb is about the most expensive (and dangerous) option that one could choose. (Although, if a person is bound and determined to run a 250w bulb *anyhow* to try to heat the whole darn coop, you may as well put it over the waterer and get double-duty out of it)
Homemade waterer-heating devices are usually 20-40w, sometimes larger. You want to make sure you have GFI protection on the circuit and the DIY thing is not made in an unacceptably unsafe way, remembering that you have electricity, heat, dry bedding, and shock-conducting water in close proximity to each other.
Heated dogbowls seem to run 60-120w.
If I recall correctly, the 2gal plastic heated waterer that I have is something like 100w; commercial heated bases for galvanized waterers are in the same ballpark. A heated bucket is 120w.
A thermostat will save considerably on energy useage in not-too-cold conditions. When the air temp. is low enough that the device has to run constantly to keep the water liquid, a thermostat ceases to matter much.
An insulating jacket put on any waterer where it's both safe and feasible will decrease electrical usage too.
In case anyone cares, I have a 2gal heated plastic waterer for the pen of 11 sussexes; for the turkey pair I have a blue heated bucket, plugged in only when it starts to freeze and then unplugged again (cuz I don't trust the thermostat not to waste me money); and the pen of 7 miscellaneous chickens has a 3gal galvanized font set atop a (filled) small heated dogbowl that is rigged to drain away any spilled water for safety's sake. All of them have worked quite fine so far down to, uh, maybe 23 F; I can't say how well the latter arrangement would work in colder temps.
Good luck, have fun,
Pat
