Even 100w more for 5 hours a day every day is like £22 a year.
It's likely running overvoltage on auto anyway, undervolting could save you 50w if you are that worried.
Easy enough to get a higher overclock on lower voltages and use less power than the defaults will for an unclocked chip.
One particular chart I just saw was a 6700k at 4.7 drawing 220w vs a stocker on auto which was drawing 192w at its default settings. At 4.6 it was drawing just 189w, so less than the stock settings because the auto voltage was so high.
While you make a good arguement it is off-topic.
spluff asked if overclocking 6700k would improve gaming performance.
The answer is no, it will not.
But it will take time and cost money.
My calculations are £30.00, so not much more. But the point is it cost more, and for gaming there is no or insignificant gain. So it's wasted expense.
Because there is no significant gain, the time spent learning how to do the overclock as well as time to do it are therefore also wasted effort.
Donate the electrical cost savings to your favorite charity.
Donate the time wasted overclocking to some disabled or old person in your neighborhood by offering to cut their grass or wash their windows.
While it's only a little money or a little time spent it will make our world a little better.
Even a selfish gaming addict can use the money toward a new game and the time gaming.
If you want to overclock for fun than do it. Just don't try justifying it by saying 'it's only £20 more a year.'