In article ,
"Simon" writes:

I've just found a DIY example at http://www.edcheung.com/automa/power.htm

I just took a look, out of curiosity. He uses the old Motorola
MC1495L 4-quadrant analogue multiplier to form the instantaneous
product of the voltage and current, just like I did in one I built
in the early 1980's. Trouble is that this chip is obsolete for 20
years now, and no one produces a 4-quadrant analogue multiplier
chip with differential inputs, which is what you really need for
this task. I suspect most true power meters nowadays work buy
sampling the voltage and current and multiplying digitally.

--
Andrew Gabriel