wrote:

"With AC, it's the sum of instantaneous products of current and voltage
over a cycle, divided by the product of their average values.
It's 1.0 if current is proportional to voltage. "

David's memory is correct. For AC, its the cosine of the phase angle
between the voltage and current. When there is zero phase shift, the
power factor is 1.0

What's the phase shift of a switching power supply? :-)

Nick