Keith Dysart wrote:
Are you really saying that if I measure the instantaneous
voltage and the instantaneous current then I can NOT multiply
them together to obtain the instantaneous power?

It certainly works some of the time.

If I can not do it all the time, when can I do it?

Actually, "multiply" is ambiguous. You need to take
the *dot product* of the voltage and current to
obtain power.
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73, Cecil http://www.w5dxp.com