Electronic – How to convert AC current at 110v to Watts


So I'm trying to measure the power consumption of my home PC. I used to have a kill-a-watt meter which I can't find, so I wired up my multimeter to measure the current.

Now I'm not sure what to do next.

Is it just .6Ax110v = 66W? Or do I have to do one of those sqrt(2) adjustments?

Seems like a silly question to ask.

Best Answer

I suggest you read this answer to a previous question.
For a resistive load

\$P = V \cdot I \$

so the 66W would be correct. No need for the \$\sqrt{2}\$; you want the RMS (Root-Mean-Square) value, not the peak value. However, your PC's power supply isn't a pure resistive load and then

\$P = V \cdot I \cdot cos(\phi) \$

where \$\phi\$ is the phase difference between current and voltage. You need to measure both simultaneously to determine the phase, something your kill-a-watt does. Since a cos is maximum 1 your power may be lower than the 66W.

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