Peakon

Meet Peakon, a plug who knows that mains voltage is always changing.

The electricity from a wall socket is alternating current. The voltage swings smoothly up to a positive peak, down through zero to a negative peak, and back again many times each second. Its average over a whole cycle is zero, so a plain average tells you nothing useful about how much power it delivers.

Instead we use the root mean square value. For a sine wave it is the peak divided by the square root of 2, V_rms = V₀/√2. A supply rated at 230 V rms has a peak of about 230 × 1.414 ≈ 325 V. A 120 V rms supply peaks at about 170 V.

The rms value is useful because an AC supply heats a resistor exactly as much as a steady DC supply of the same rms voltage.

So the number printed on an appliance is the rms value, even though the voltage briefly climbs well above it twice in every cycle.