A very general rule of thumb: Small, single-PV-panel systems that produce about 75 watts can cost about $900 installed, or $12 per watt. A 2-kilowatt (1 kilowatt=1,000 watts) system that meets most energy needs of an extremely energy-efficient home can cost $16,000-$20,000 installed, or $8-$10 per watt. A 5-kilowatt system that completely meets the energy needs of many conventional homes can cost $30,000-$40,000 installed, or $6-$8 per watt.
first reference:
The $1 per watt price point has been a goal of the solar industry for a long time: First Solar announced that "it reduced its manufacturing cost for solar modules in the fourth quarter to 98 cents per watt". It’s a bit unfortunate that it is happening to First Solar at a time when the economy is in bad shape and fewer people and corporations can afford solar panels, but it is nonetheless a great achievement.
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References :
http://www.treehugger.com/files/2009/02/first-solar-inc-manufacturing-costs-solar-panels-below-1-dollar-watt.php