According to a Lawrence Berkeley National Laboratory report, the cost of solar photovoltaic systems in the United States fell 30% between 1998 and 2008.

They found the average installed costs, in 2008 dollars, fell from $10.80 per watt to $7.50 per watt during the 11-year period.Small PV systems, typically used on homes, saw their costs fall the most over time.

Costs varied by region, too — in 2008 Arizona had the lowest, at $7.30 per watt, with California at $8.20 per watt. Pennsylvania and Ohio were the most expensive, with average installed costs at $9.90 per watt.

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