Convert Power to a Peak Voltage, when to multiply by sqrt(2) - Not so simple?
In terms of watts, the relevant quantity is an average value. RMS watts make no sense. RMS volts make sense and RMS volts x RMS current equals average power and nothing else. If you want to know the power peak level and the voltage and current are sinwaves that are in phase, the peak power is twice the average power. If volts and amps are not in phase then you can’t estimate the peak power from the average power without knowing the phase angle.

Please read this for confirmation that RMS power is a misnomer. And here are a couple of examples of power waveforms when V and I are in phase and when they are not in phase: -



Note that the power waveform shape and size remains the same but its average level falls when V and I are not in phase. Picture source