To be honest, all x86 processors are reasonably efficient if you pick the right power points to run them at. My experience with my laptops is that windows will arbitrarily use 2-3x idle power draw while on battery for no apparent reason. There is no reason for a CPU to be running higher than idle the vast majority of the time in a laptop, for tasks that you would be using a laptop for on battery.
I think most laptops are just poorly designed and Windows isn't reliable about using minimal power. MFGs make big laptops with 60whr batteries (instead of 100wh), they put bloatware on them that consume excess power at idle, MS Windows doesn't reliably ensure low power consumption on battery (seriously, I have to hibernate/shut down my laptop when I stop using it, otherwise it will consume its entire battery in 8hrs during sleep mode, somehow), and so on. Most problems with Windows laptops aren't Intel's fault, IMO.
I agree that Intel really should clarify, at least, the difference between their "Celeron" class (e-core only) and "Core" class processors (with P cores), since there's such a massive difference in performance capability between them.