Your heat transfer calcs are correct. Without knowing the final design though, any calculations we make are just guesses and probably do not take into account some important design factors.
Reading the patents may help give a little insight into the complex workings of the unit (link in ibox). You're most likely correct when wondering if there are some other factors at play (i.e. energy storage when not in use, residual heat transfer from magnetrons cooling, multiple microwave frequencies).
I'd love to know the final design and be able to run the calculations, but we don't have that information.....yet.
Go WDRP!!
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