Resistor Power Rating and headroom for AC circuit
For determining the power a voltage will dissipate when applied to a resistance, use the RMS voltage.

One way to look at the RMS voltage is that's the equivalent AC to the DC voltage that would dump the same power into the same resistance.

For example, 5 V across a 2 &Omega; resistor will dump (5 V)2/(2 &Omega;) = 12.5 W into the resistor.  That is true whether the 5 V is DC, or 5 V AC RMS.  Actually, the voltage in the equation is always RMS.  For DC, the RMS is the DC voltage.