Until last summer, Illinois gave homeowners and businesses as much as $5,000 on a $10,000 solar system. Solar panels can supplement natural gas heat, which got more expensive last year.

But the program was temporarily halted as the state waited to learn more about how a new federal tax credit for solar energy would be structured for 2006. The state wanted the two programs to complement each other.

Now the state is starting its rebate program again, even though it doesn't have all the details about the federal credit, according to Hans Detweiler, deputy director of energy and recycling for the Illinois Department of Commerce and Economic Opportunity.

"We might have to hold our breath for a long time" to wait for publication of the new tax credit rules, Detweiler said. "We've decided to go ahead with the best available information."