You cannot stop there. You must go to the next Physics course. You have to account for the walls and furniture. Just like a freezer, which works better/faster/cheaper when full, than when empty, your walls and furniture are like the packages of food in a freezer, and they cool off with the house temp. It takes longer…

You should think of your house in the winter like a hot air balloon - surrounded by cold air, and the hot air constantly escaping. The colder the outside air and the higher the setpoint inside, the more heat you're throwing in there to maintain a given temperature.