When you have AC in a circuit, the electrons are being pushed forward for a fraction of a second, and then backwards for a fraction of a second. Between each of those movements, there’s a moment that the force is slowing down and reversing direction, and it’s not accomplishing much.

If AC power is spending some of its time at 170 volts, and some of its time at 0 volts during the transitions, we need a way to figure out how the power output compares to a steady voltage in one direction. Because power is proportional to voltage ***squared***, we look at the voltage squared over time, add it all up, and take the square root of it to find the equivalent voltage.

Looking at it very roughly, 120^2 + 120^2 (constant voltage at time #1 and time #2) is very close to 170^2 + 0^2 (varying voltage between a max at time #1 and nothing at time #2). So the total power of a sine wave with a max of 170 voltage is close to a flat line at 120 voltage.

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