Hello! OpenMC probably can’t give you the number you want in Watts because by definition, Monte Carlo can only ever give you a shape of the flux, not a magnitude. So it can give you a power distribution that you normalize to the total power you set.
You can define power with several different tallies. I’ve had good success using simply the “fission” tally, but you can also use “kappa-fission” or “fission-q-recoverable” depending how sophisticated the assumption you want. In my experience, I’ve used a known normalization factor of 66945.4 W for a fuel rod based on the power output of an entire reactor divided by the number of rods in the reactor.
For a cool example of changing temperatures and water densities in-memory in OpenMC, check out this notebook: Jupyter Notebook Viewer