Roy Lewallen, W7EL wrote:
"The average power is therefore relatively small, much smaller than the
product of RMS volts times RMS amps."

RMS is short for root-mean-square. RMS is synonymous with the "effective
value" of a sinusoidal waveform.

Therefore, the average power for the time period of one complete cycle
or any number of complete cycles is the product of the effective volts
times the effective amperes.

See page 19 of "Alternating Current Fundamentals" for derivations of the
proof.

Average power is exactly the product of rms volts times rms amps in
usual circumstances.

Best regards, Richard Harrison, KB5WZI