"During S-Parameter Simulations MWS monitors are normalized to 1W peak (=0.5 W rms) input power at the port." Source: https://www.cst.com/Content/Events/UGM2007/10-Becker2.pdf
Also the figure below of a patch antenna I had simulated shows the Power stimulated as 0.5 W. It can be seen in the ID results section under the heading "Power."