Temperature control using a water bath system

Q = m c dT

where Q=energy, m = mass (of water) c = specific heat (of water =1 in SI), dt is your temperature rise from the initial temp to the 50 degC.

Divide dT over t (time) you'd like to heat it up, and divide by the same t the Q, and you have energy over time, i.e., power. Not considering losses, of course.

How much to maintain it? Depends on the reservoir losses. After heating, a really small amount of energy is required to maintain it if well isolated.