How do you know how much electricity a well-designed core can generate

May I ask what tally OpenMC can use to obtain the power of the reactor

Hello! By definition, Monte Carlo cannot give you a power in Watts because Monte Carlo only calculates the shape of the neutron distribution. However, if you know what overall power you are looking to achieve, you can use that to scale to your calculation. You may find the following discussion interesting: Finding Power from tally - #2 by mkreher13

Thank you very much. I will give it a try