Reading Keith`s posting, somebody wrote:
"The power is Vavg"Iavg"cos(theta)"

This is mistaken. The average value of a sine wave is 0.637 times the
peak value. We use 0.707 toimes the peak value of a sine wave, the rms
value, which is the effective value, that is, it is as effective as d-c
in making power calculations.

The average power must be the same as d-c. This is peak volts times peak
amps divided by two, or 1/2 the peak power. 0,707 x 0.707 = 0.5, so we
use rms, not avg. volts and amps to calculate average power, which is
the same in capability when producing light and heat from resistive
devices.

Best regards, Richard Harrison, KB5WZI