State and Federal rebates and tax benefits.

The California Energy Commission Emerging Renewables Buy Down Account offers cash rebates on eligible photovoltaic systems: $4.00 per watt, or up to 50 percent of the total eligible system cost, whichever is less.
For example, for a 2kW system costing $20,000 installed. the rebate amount is approximately $8500 (the $4.00 per watt rebate applies because it is lower than 50 percent of the total cost).

The Federal Energy Policy Act of 2005 established a 30% tax credit for the purchase and installation of residential solar electric, after rebate is calculated.
For the 2kW system, you'd get a tax credit of 30% on $11,500, or $3,450.