A circuit’s voltage is an indicator of the force behind the current in that circuit. To talk meaningfully about that voltage (and current), we need terminology and a way of looking at electricity values to understand and compute the things we want to know—like power.
With alternating current (AC) electricity, we need a way of describing the electricity that will hold true throughout our calculations. In the U.S., the electricity “standard” (the effective voltage) is 120 volts AC (VAC) with a sine waveform (a graph showing the variation of the voltage measurement against time). But the peak voltage value is about 170 V. This peak occurs only for a moment—during each positive and negative swing of the waveform. Unfortunately, the effective voltage can’t be calculated by using an average—with AC electricity, the average is always zero, since the positive and negative portions of the waveform cancel each other.
Instead, the effective voltage is described by calculating the square root of the mean (average) of the voltage squared. This is where the term “root mean square (RMS)” comes from. You square the peak voltage, divide that squared number by two, and then take the square root of that average. This is easily done for a true sine waveform.
1702 ÷ 2 = 14,450 (the mean of the square)
√14,450 ≈ 120 VAC (RMS)
Now we have a number that we can use practically and meaningfully in our calculations. For example:
120 VAC (RMS) × 1 A (also RMS) = 120 W
When measuring RMS voltage and current in an AC waveform, quality meters do these precise conversions from peak to RMS instantaneously. These meters are also able to compute and display true RMS figures, even when the waveform is not precisely sine wave, or if the negative and positive peaks are not at the same voltage. These are much deeper calculations than we can go into here.
Less expensive meters use a simple algorithm to estimate RMS voltage. That algorithm holds up fine if the sine wave is pure or very close to pure, so for people who are measuring household currents without the need for true RMS, the cheaper meters will work just fine.