This is an estimated cost, per watt of rated system capacity, for materials and labor to install a complete grid-connect solar electric system. This price does not include any state or federal incentives. Rated system capacity is in PTC (not STC) watts. Remember that higher efficiency panels are more expensive per installed watt than lower efficiency panels.
Researchers at the Department of Energy's Lawrence Berkeley National Laboratory (LBNL) conducted a study ("Tracking the Sun: The Installed Cost of Photovoltaics in the U.S. from 1998-2007," by Ryan Wiser, Galen Barbose, and Carla Peterman) that examined 37,000 grid-connected PV systems and found that average installed costs per rated watt is $7.60 in 2007 U.S. dollars. Green Building Studio defaults the Installed Panel Cost to a rounded value of $8.00. The same LBNL study found that total after-tax incentives for commercial PV were $3.90 per watt in 2007 U.S. dollars. Total after-tax incentives for residential systems averaged $3.1 per watt in 2007 U.S. dollars. If you anticipate applying for any incentives for the installation of the system, adjust the Installed Panel Cost accordingly. The following website has information on available incentives for renewable energy systems in the United States: http://www.dsireusa.org/.