The costs are often high, making incentives critical. For instance, a solar-power-based 500-kilowatt system would cost about $3.5 million based on an estimated cost of about $7 per watt to buy and install it. California's credits can cut that cost by about $1.5 million, and federal tax credits would reduce it by another $1 million or so, according to Rhone Resch, the president of the Solar Energy Industries Association, a Washington, D.C.-based industry group.

Resch estimates that the bottom-line cost of system would be about $1.3 million.

Any return on investment depends on how much a user is paying for electricity, although Resch said a six- to eight-year payback for a large user would not be unusual. And since a business would likely finance the project, it could generate a positive cash flow from the investment immediately, he said.

Another green approach was taken by Alan Hirsh, executive vice president of Sea Gull Lighting LLC in Riverside, N.J. Last fall, he completed a 500-kilowatt system that provides 20 percent of the power needs for a 500,000-square-foot warehouse Monday through Friday. On weekends, the panels can provide all of the power needed. The solar panels take up about 60,000 square feet on his warehouse roof.

Hirsh's company is using SunEdison LLC of Baltimore, a solar services company that installs the system and continues to own it. Sea Gull, however, pays a rate for the electricity it uses that's below what it would pay if it bought from a local power provider, he said.