If you had a super-insulated vessel, 1000W will bring 50 gals to a boil. 100W would, given enough time.
But you sustain real time heat loss with that huge SS vat filled with water. It will radiate heat and quite a bit. the higher the temp, the more it radiates out, and at some point, as fast as the element puts it in. I don't know at what temp that would settle at, but I doubt it being even anywhere near 120F.
Insulating that vessel well would be your first step. Then you should estimate heat loss at a given vessel temperature and (its surronding) ambient temperature. Then you would know how many BTUs (or Watts) to input to overcome that heat loss.