From an article by Chris Clayton in the Progressive Farmer:

USDA released a study Friday showing 8,569 farms nationally had either solar panels, wind turbines or methane digesters in 2009.

The study highlighted at the USDA Outlook Forum looked at the costs savings of farms producing their own energy, as well as the costs of building such facilities.

One clear takeaway from the study is that the vast majority of solar panels, methane digesters or wind turbines being used for on-the-farm power were build since 2005.

Of the three major renewable-energy sources drawn from the survey, solar power by far the largest renewable power source being used on farms. USDA showed 7,968 farms reported using photovaltic or thermal solar panels in 2009. California was the largest state in terms of farms reporting panels, and was also far and way the largest state in terms of the number of solar panels.