I wrote a long boring post on "effective aperture". I apologize. I
regret not becoming familiar with Kraus`s "Antennas" long ago. Kraus
makes many ideas clear with few words.

On page 43 of his 1950 edition of "Antennas" Kraus writes:
"The ratio of the power W in the terminating impedance to the power
density of the incident wave will be defined as the effective aperture
Ae.
Thus Ae = W/P
If W is in watts and P is in watts per square meter, the Ae is in watts
per square meters."

Best regards, Richard Harrison, KB5WZI