ROOT MEAN SQUARE (RMS) VALUE. The value of an AC voltage is continually. changing from zero up to the positive peak, through zero to the negative peak and back to. zero again. Figure-1: Difference between peak and RMS voltage. Clearly, for most of the time it is less than the. peak voltage, so this is not a good measure of its. real effect. Instead we use the root mean square. voltage (VRMS) which is. 1 2≈0.7 of the peak.