To determine peak voltage from an oscilloscope trace, the maximum vertical deviation of the trace from the centre line is measured from the screen. This number of divisions is multiplied by the Y ...

At other times it is less! To calculate the r.m.s. voltage from the peak voltage the factor of \(\frac{1}{\sqrt2}\) is used. Peak power is twice the r.m.s. power. \(Power=current\times voltage\).