Houses are Energy Systems

The house acts as an energy using system with these components:

The windows of your house affects most of these.

The space heating and cooling are affected by the outside temperatures and wind speed, the temperature being maintained inside the house, the amount of insulation in the walls, roof, and floor, and the performance of the windows and doors.
The lighting that you need to add is affected by the number of daylight hours, and the amount of daylight coming in through the windows. This depends on the size, number and location of the windows.
Humidification and de-humidification are affected by the outside temperature and dryness, the moisture production inside the house, and the relative humidity that you require to be comfortable and healthy. The windows and doors can leak the humid air out in winter or can be used to ventilate unwanted humidity especially from the basement. The other aspect of humidity is controlling the condensation on the cold surfaces of the windows.
Ventilation is a comfort and health concern and can be achieved mechanically or naturally through the windows and doors.

Residential energy use in Canada breaks down like this:

House Energy Use

In addition to direct energy costs, choosing Energy Efficient windows can affect:

The size of heating/cooling system. With an improved building envelope you may be able to use a smaller heating and cooling system. This will save money when building or when replacing the old furnace.
The location of the heating registers. These can be placed along the inside walls rather than the outside, saving ducting, drapery problems, and basement headroom. This can save money when building.