There are two major incentives:
For residential photovoltaic systems the rebate is $0.35 per watt by SCE’s (our area) calculations. For new homes the rebate is handled through the California Energy Commission. These rebates will be going down as more rebate reservations are made. Please see the “trigger tracker”. There is also a 30% federal tax credit
The new million solar roofs or California SB1 is a rebate for installing solar electric (photovoltaic) systems or SDHW systems on homes with electric water heaters. At the moment (4/08) it is stuck in a “pilot program” in San Diego.
You can get a good idea of what a system will do by clicking on the clean power estimator at the right. Put in you zip code. It will then open up a page to build your system. For starters put in $9,000 per kilowatt, set the PV adjustment to 0, and the annual escalation to 5% . The rest of the info you will know and can play around with. It will give you an idea of payback and how much CO2 a system will offset per year.
There is 30% tax credit plus a five year accelerated write off for commercial with 50% allowed in 2012.