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.

1.5kW is a nice average for cooking a meal, rather than reheating something
in the oven. The peak output could be as high as 10kW for Christmas dinner.

Christian.