On Jan 12, 2:08 am, Roger Sparks wrote:
I learned long ago that it DOES have a precondition. The power measured will only be resistive if measured across a pure resistance. If the measurement is across resistance plus reactance, then reactive power is also measured so P(t) = V(t) * I(t) becomes the sum of an unknown mix of two or more waves.

In power engineering, the power can consist of two components, active
power, V*I*cos(A) = watts, and reactive power, V*I*sin(A) = VARS.
These terms are defined in The IEEE Dictionary. P=V*I results in
Volt*Amperes, part active and part reactive if the voltage and current
are not in phase.
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73, Cecil, w5dxp.com