The Thermos principle, a metaphor for the high energy efficiency of a Passive House

The “service” that needs to be provided in this example is keeping coffee warm.

This task can be achieved either by applying energy (hotplate on the right) or by avoiding the loss of heat in the first place (thermos flask on the left).

While there is still minor heat loss, it is so small that there is no need for the hotplate.

It becomes clear why energy efficiency not only “saves some energy”, but can also replace the provision of energy to a great extent.
Dr. Wolfgang Feist, Passivhaus Institut

The completed facade on the right with insulation and plaster; Background: a uninsulated building, partly hidden by trees.
passipedia.passiv.de