Why do you have to find RMS value?

Root mean square (average *) voltage

Because ac (alternating current) does not warm up a lamp, heater, etc. as well as a direct current of the same value as the peak ac, we need to work out how to find the equivalent dc (direct current) voltage for the ac situation.

The r.m.s. value of an alternating current or voltage is the value of direct current or voltage which produces the same heating effect.

For a sine wave, the r.m.s. value is 0.707 times the peak value, or, conversely, the peak value is 1.414 times the r.m.s. value.

Peak voltage

Peak voltage is measured from the zero axis to the top of the curve. So, in the case of the UK mains supply, where the average r.m.s. voltage is 230 volts, Vpeak = √2 x Vrms = √2 x 230 V = 325.22 volts.

Note: The average voltage is always zero.

* The average value and the r.m.s. value of a sine wave are not the same thing. The average value of a sine wave is 0.636 x the peak value.