When I boil pasta, I use full heat to get the water boiling, drop in the pasta, then let it come back up to a boil, then drop the temperature as low as possible to keep a slow boil going--about medium-low.
Guesstimating that the boil takes about as long as the time to cook the pasta, I'd say that the pasta cooking portion only takes up about 1/3 of the energy of the total (cold pot to cooked pasta).
The 80% savings only covers the pasta cooking phase, so overall, it's only saving 4/15 or 27% (roughly). If you use a lot more water, then that initial boil time further reduces your savings.
The asterisk: water boiling phase excluded
When I boil pasta, I use full heat to get the water boiling, drop in the pasta, then let it come back up to a boil, then drop the temperature as low as possible to keep a slow boil going--about medium-low.
Guesstimating that the boil takes about as long as the time to cook the pasta, I'd say that the pasta cooking portion only takes up about 1/3 of the energy of the total (cold pot to cooked pasta).
The 80% savings only covers the pasta cooking phase, so overall, it's only saving 4/15 or 27% (roughly). If you use a lot more water, then that initial boil time further reduces your savings.