ee484,
For simple sinusoidal waveforms, Power is the RMS voltage times the RMS current component that is in-phase with the voltage. P =VICos(Theta), where theta is the phase angle between voltage and current. As an extreme example, consider a purely capacitive load. In this instance, the current leads the voltage by 90 degrees. For 1/2 of the cycle, the capactor is drawing current from the source. For the other 1/2 of the cycle, the capacitor is supplying current to the source. The net power is zero (Cos[90 Degrees]) = 0.
.
VA is simply the product of the RMS voltage and RMS current.
.
For the example you give, the phase angle between the voltage and current is the arcCosine of 350/500.
.
Non sinusoidal waveforms can be analyzed by doing a Fourier analysis of the voltage and current waveforms, analyzing each frequency component separately and adding the results.
Regards,
Kral