CST - RF Power Calculations from Waveguide port signal (PIC Simulations)

In one of examples i.e. Magnetron Design, the out put power has been calculated using the formula (Power in Watts = Square of output / 2 ); just wanted to confirm why this division by 2 is done for calculation of port power ? Can anyone suggest a correct formula or elaborate this one.. ? (https://space.mit.edu/RADIO/CST_online/mergedProjects/EXP_PD/examplesoverview/pic_solver/magnetron.htm​​​​​​​)

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