I'm Italian, so by definition I know how to cook pasta (JK ;)).
I keep it rolling boil for the whole time. To prevent spaghetti to stick, you have three helping procedures.
The first is: you break them in the middle. This is heresy in Italy, but I find it a good solution to prevent clumping and to fit the spaghetti completely into the water from minute one.
The second trick is to clump the spaghetti together using the index and the thumb of each hand, in the center of the spaghetti, shaping the fingers in a ring. Then you rotate these rings in opposite directions. the spaghetti open in a sort of double cone, and the spread prevent them to clump. Then you throw them in the boiling water.
Third, you actually continue mixing them for the whole six minutes.
Regarding energy, the heat you "waste" actually goes into your house, so you are actually heating yourself, and in winter is good. In summer, I personally tend to eat something different.