RMS

Think about it ......... at what point on an AC sine wave do you rate the voltage (current or power for that matter)  is it at Peak or when the wave crosses the origin (0 volts) ?

The voltage is changing 60 times a sec. (60Hz)

The effective voltage is the sum of all the instantaneous voltages across the cycle. This is called RMS root mean square.

RMS is not the average value but the effective value in other words the amount that does useful work.
 

Here is the deal ......... if you buy a stereo system and the salesperson says the unit outputs 100Watts_pp and another says that his outputs 90 W_rms which one delivers the most power?

Well ...... 100Wpp = 50p(peak)  which is 50W x .707 = 35Wrms   get it now!!

many a fool has fallen for the Peak power sometimes even Peak to Peak power!

RMS values are the effective values and the ones an honest person uses.
If the voltage amperage or power are given and RMS is not mentioned assumed that the values are RMS.

trivia .......... what is the peak voltage of mains ????

Vrms = Vp x 0.707
 

Vp = Vrms/ 0.707

= 120V/0.707 = 169 volts

and more startling is the Vpp = 2 x Vp = 169 V x 2 = 339 V

so the next time you get a jolt remember so an instant the voltage was over 300 volts!!!