I am not sure what your search for "simplest circuit" is as there are many defintions I would need to know first, e.g AC vs DC, power level, etc. to narrow it down.

Like others mentioned it is a about measuring voltage*current.

I found this video for low power DC measurements helpful as well https://www.youtube.com/watch?v=LUB8RWzzLWc&t=15s or for AC Power Meter https://www.youtube.com/watch?v=QXNlsawaYEg&t=868s