RMS Voltage
Definition
The ‘effective’ voltage of an AC supply — the DC voltage that would deliver the same power.
Symbol: · Unit: V
Technical definition
Root-mean-square voltage: V_rms = V_peak/√2 for sinusoidal AC. Power calculations use RMS values: P = V_rms²/R. Mains ratings (120 V, 230 V) are always RMS.
Example
US mains at 120 V RMS actually peaks at about 170 V each cycle — the 120 V figure is the equivalent steady heating value.
Common confusion
RMS is not the average voltage — the average of a sine wave over a full cycle is zero! RMS squares first (making everything positive), then averages, then takes the root. That is why it is the meaningful ‘effective’ value.