In 2014 we joined a training program with SCE for new rebates. The class revolved around software made by a company named Wrightsoft.  It specialized in load calculations on residentail homes to properly sized the HVAC equipment to give maximum efficiency and 100% capacity, coupled with proper duct design, to each residence. SCE found most homes in Ca the equipment was oversized and the ducting was undersized. 

Contractors are able to draw the house out, add the window types, wall construction,the insulation R value of them as well as the attic space etc. Even the orientation of the home since it is important to know what windows face the morning or afternoon Sun.

The end result is an HVAC system that makes the home more efficient, and more comfortable by minimizing temperature swings (otherwise to warm to too cold) allowing the system to run longer cycles. The longer cycle times also reduces the starting of the compressor which draws a lot of energy and causes more wear and tear shorting the life of the system. 

Not all contractors practice this technology and either use the old "rules of thumb" such as 500 sq ft to the ton, (a ton being 12,000 BTU of cooling) or just oversizing to cover themselves. 

The program is also used to properly sized the ducting for the given system based on the CFM or airflow of the ducting. Proper sizing of the ductwork is essential for the system to utilize 100% of its capacity. For example a 3 ton system (36,000 BTU) with undersized ducting may only put out 75% of those BTU's. In the end you are not getting what you paid for and unfortunatly you are paying more to operate it than you should. 

For more information on Manual D duct design and proper sizing click Here