Wattlet

Meet Wattlet, an electric meter who knows that voltage and current do not always march in step.

In an alternating current circuit, the voltage and the current both swing back and forth many times a second. In a plain heater they rise and fall together. In a motor or a fluorescent lamp the coils make the current lag behind the voltage by a phase angle φ.

Only the part of the current that is in step with the voltage delivers real power. The average power is P = V_rms I_rms cos φ, and cos φ is called the power factor. It is 1 for a heater and less than 1 for anything with coils or capacitors.

A motor on a 230 V supply drawing 10 A with a power factor of 0.8 uses 1840 W of real power, not the 2300 W the simple product suggests. The rest of the current just sloshes back and forth.

That sloshing current still heats the cables, so power companies ask big factories to fix poor power factors, often by adding capacitors that bring the current back into step.