I still have an old 3930K which is 6 cores at about 3.6Ghz with 16GB of RAM, it was used as a game server for years but its not on now. It consumes about 110Watts at idle, there is no GPU of note (710GT or something like that) but its mostly all the CPU and lack of power control. A newer desktop however with 32GB DDR5 and a 9800X3D will idle at 40Watts with a lot more drives and a modern GPU etc. New machines use considerably less power at idle and when you go back as far as a Pentium 4 those things used 100Watts all the time just for the CPU whether in use or not.
Anything since about the Core 9th gen does behave fairly well as do all the modern era Ryzen processors. There is definitely some CPUs in the middle that had a bunch of issues with power management and had performance issues ramping clockspeed up which was felt on the desktop as latency. Its been for me a major advance of the past 10 generations of CPUs the power management behaviour has improved significantly.
Eh, your CPU can't stay eg in the high power state (post its steady state thermal envelope) for very long, but you'd still like to know how much power that consumes.
The kill-a-watt is unlikely to be fast enough. Especially if there are perhaps capacitors in your computer's power supply?
Are you interested in how much energy a certain instruction uses or are you interested in how much power your computer uses while running a certain program?
Anything since about the Core 9th gen does behave fairly well as do all the modern era Ryzen processors. There is definitely some CPUs in the middle that had a bunch of issues with power management and had performance issues ramping clockspeed up which was felt on the desktop as latency. Its been for me a major advance of the past 10 generations of CPUs the power management behaviour has improved significantly.