How to calculate Microwave Power

-If you have a magnetron produce __# of microwaves, each with frequency
of __ MHz, how can one calculate the power of those microwaves (in watts)?

-the microwaves have electric field and magnetic field associated with
them, taking just one microwave (in the waveguide) and its associated electric field,
if you had another microwave in phase, does the electric field
simply double?

Thank you.