High-performance windows not only provide reduced annual heating and cooling bills; they reduce the peak heating and cooling loads as well. This has benefits for the homeowner, in that the size of the heating or cooling system may be reduced, and it also benefits the electrical utilities, in that load factors are reduced during the peak times in summer.

The peak load for a building is the maximum requirement for heating or cooling at one time. These loads determine the size of the furnace, heat pump, air conditioner, and fans that must be installed. The table illustrates typical reductions in cooling peak loads that occur with different window types used in a typical house. The consumer can benefit directly from peak load reduction, because heating and cooling systems can be sized smaller, resulting in initial cost savings in some cases. Test homes have been built in various U.S. climates (Las Vegas, Phoenix, Lakeland) have demonstrated significant peak demand savings as a result of using efficient windows. Recently, several houses were built in Las Vegas with energy efficient improvements including spectrally selective windows. The total size of the air conditioning system was reduced by 30 percent.

Note: these peak loads were calculated using RESFEN for a typical 2000 sq ft house with 300 sq ft of window area located in Phoenix, Arizona. Source: Residential Windows by Carmody, Selkowitz, Arasteh and Heschong, 2000.