Thanks for running the numbers. The important question is how many personal computers are currently switched on at this exact moment? What's a small improvement to one person can make a huge difference nationally. Just because there are other devices that use yet more energy still (e.g. kettles, fridges, vacuum cleaners, etc.) doesn't mean there shouldn't be a drive for increased efficiency in all our electric devices where possible (in my personal opinion).
Then keep in mind that a "30 W" computer will use much less than that in idle, where it'll be for 90+% of the time. Including auto-sleep, which Macs are good at, you'll be well below 10 W on average.
> doesn't mean there shouldn't be a drive for increased efficiency in all our electric devices where possible (in my personal opinion).
Absolutely, but replacing one energy-efficient sub-30 W CPU with another slightly more efficient CPU won't do huge things to the country's energy bill.
