Steve Nosko wrote:
. . .
Power, on the other hand, cannot be directly measured it has to be
calculated. Power, we know, is Volts times Amps or E x I. There are other
ways to figure out power which I won't go into here, but this is how we must
be satisfied doing it.
. . .

Power meters are in wide use and directly measure power by detecting the
amount of heat it produces.

Roy Lewallen, W7EL