# How does one measure the power factor?

How does one measure the power factor?

$$\text{power factor}\equiv\frac{\text{power}}{|V||I|}=\frac{R}{\sqrt{R^2+(1/\omega^2C^2)}}$$

for an RC circuit driven with $V(t)=V_0\cos{(\omega t)}$.

• Watt meters measure power (as in watts) not power factor. Names have clues! – Andy aka Apr 14 '15 at 16:50
• @Andyaka: Sure, but they're often found in the same device (e.g., in the Kill-a-Watt meters). – Geremia Apr 14 '15 at 16:55
• Expanding out the part named "power" in the equation will get you the answer. – Ignacio Vazquez-Abrams Apr 14 '15 at 17:21
• Here's a quick Google search: electrical4u.com/… It looks like they compare the phase of the voltage and current at a point in the circuit. – Greg d'Eon Apr 14 '15 at 18:41
• Try @Gregd'Eon's link for the electromechanical type. The electronic type samples voltage and current much faster than the mains frequency so it can calculate real and apparent power from the current samples, the voltage samples, or the product of each current sample with each voltage sample (discretized instantaneous power). – Spehro Pefhany Apr 14 '15 at 18:58