[eru]

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?

[immibis]

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?