Simulate EM wave with known power output

Dear Sim4Lifecommunity,
in an EM FDTD simulation, I am trying to model a situation in which I have microwaves of known frequency (915MHz) and heat up a phantom. It is heated up by a machine that has a known power out (e.g. 12W). In the simulation however, I can only enter a resistance and a voltage. Is there a straightforward way to simulate such a situation, or how do I calculate the correct power output from the voltage?
I am already aware of the manual (especially the chapter "Power Waves"), and have tried to calculate the power from the provided theory, but so far, my attempts have not been successful. Here is one of my attempts:
I used the Thevenin equivalent and calculated Z_i=(U_0^2/P)Z_0, where U_0 and Z_0 are the quantities that can be entered into the simulation, and P is the EM input power that can be found in the analysis. Having the internal resistance Z_i of the system, I can enter it into the equation to predict a correct power output P with other values of U_0 and Z_0. Yet the result never matches my predictions.
Can someone help me?

After your simulation is done you can scale the output everything to the output power your machine has. In the analysis tab of Sim4Life you would do this by checking the normalize results (for any given field sensor) and then choosing the option "input power" which in your case would be the output power of the machine. Hopefully this helps.