October 20, 2009 | 9:02 pm

The average cost of solar photovoltaic power systems in the U.S. plunged more than 30% from 1998 to 2008, with a 4% drop between 2007 and 2008, according to a new report from the Lawrence Berkeley National Laboratory.



But a simultaneous drop in total after-tax incentives for photovoltaics from 2007 to 2008 resulted in a slight rise in net installed cost, according to the lab, which is run by the Department of Energy.



Overall net costs for residential solar systems were up 1% in 2008 compared with the previous year, averaging $5.40 per watt. Costs for commercial photovoltaics averaged $4.20 per watt, a 5% increase from 2007.



After-tax incentives for residential systems were at a historic low of $2.90 per watt in 2008, while incentives for commercial photovoltaics were at $4 per watt, down slightly from the 2006 peak.



But excluding the incentives, installation costs dropped recently after a multi-year plateau due to the solar industry’s expanded manufacturing capacity and the pressures of the financial crisis.



The early end of the decline, from 1998 through 2007, was caused by shrinking costs of labor, marketing, overhead, etc.



The Berkeley Lab study considered 52,000 photovoltaic systems in 16 states. The average cost of installation dropped from $10.80 per watt in 1998 to $7.50 per watt in 2008, or a reduction of 3.6% per year.



Small residential solar systems completed in 2008, producing less than 2 kilowatts, cost an average of $9.20 per watt, while large commercial photovoltaics producing between 500 to 700 kilowatts averaged $6.50 per watt.



The cost of going solar varies widely across states. For systems producing less than 10 kilowatts that were completed in 2008, costs range from a low of $7.30 per watt in Arizona, to a high of $9.90 per watt in Pennsylvania and Ohio. California’s average is $8.20 per watt.



But the report suggests that costs could be driven even lower through large-scale implementation.



-- Tiffany Hsu