What is the biggest factor in determining how much energy a builing uses?

Question

Is it the fabric/form of the building itself e.g. U-values of materials and amount of glazing, or is it the efficiency of the systems installed e.g. heating/cooling systems?

Answer 1

The size of the building and the climate are the biggest factors. The next biggest factor is the construction details including insulation, number, size and design of windows and doors, and the amount of air that is leaked or intentionally exchanged with the outside. The efficiency of the heating/cooling systems would probably be a smaller factor than the sum of all of the construction details. Of course, if either the construction details or the heating/cooling system is very poor, that could become the biggest factor.

In addition to the above factors related to heating and cooling, illumination is another important factor, but not as important as heating and cooling.

Answer 2

For most buildings, HVAC is your biggest energy user.  That is all affected by building construction, location, glazing, etc.