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The oscilloscope I used (Fluke 123) displays the RMS voltage directly on the screen. Since I knew the load resistance, I calculated the power dissipated in the resistor using P = (E^2)/R. Implicit in this equation is a calculation of the current. It goes:
I = E/R and P = E x I so P = E(the voltage) * (E/R)(the current) = E*(E/R) = (E^2)/R.
If you know the voltage and resistance this equation is easier to use since you don’t have to calculate the current.
Hope this helps,