There is a maximally efficient way to operate buildings when people are not in them. This is some function of building size, ambient air temperature and amount

There is a maximally efficient way to operate buildings when people are not in them. This is some function of building size, ambient air temperature and amount of time people will not be in the building (and probably other things). The key point is, unless the ambient air temperature is close to 20C, it is unlikely turning off heating / cooling completely is going to be maximally efficient.