When it comes to power, the direction of the current does not matter. With a 50 Hz AC supply, the power dissipation fluctuates at 100 Hz. In other words, there are two power cycles in every one current or voltage cycle.
We can also talk about peak power and average power. To calculate peak power, use peak voltage and current values!
For average power, make sure to use the rms values!
Or you may want to remember that the peak power is equal to two times the average power (for sinusoidal AC).