The message
from "Christian McArdle" contains
these words:

Now start cooking at about 1.5kW. You are now shoving in 2.5kW which will
rapidly heat the room by 1.5kW excess.

That assumes the 1.5kW is constant. In the case of a modern oven it'll
only be for a few minutes at the outset, and again every now and again
to keep it hot. In the case of boiling something, again you'd be using
that much perhap to raise something to the boil but with a lid on you
don't need anything like that much to keep it boiling. If the extractor
fan's running and removing steam that's much of your energy gone
straight away as water vapour carries a lot of heat.

--
Skipweasel
Pay no attention to that man behind the curtain.