How many years does it take solar-power systems to pay for themselves, and how much carbon dioxide do they offset?

First, let's lay out the numbers involved. A 4,000-watt solar-power system, a typical size for many homes, costs roughly $20,000 after a 30% federal tax credit and can meet about three-quarters of many families' electricity needs. By one estimate, it cuts carbon dioxide output by an equivalent of three-quarters, or about 7 tons per year. (That assumes a household needs 11,340 kilowatt-hours of electricity per year and gets 7,680 from its 40-panel system.)

As for payback time, it depends on where you live. Some areas, such as the Southwest, offer stronger, more consistent sunlight and make solar systems more effective. Meanwhile, some states offer tax credits and incentives for solar systems while others don't. Plus, many people get long-term financing from lenders for solar electric systems so they are cash-flow positive from the start. For example, they pay $100 a month for a lender-financed solar system that saves them $110 a month in energy.

Here's a general guideline: In states with hefty rebates, like California and New Jersey, the payback period is seven to 10 years. It may be twice as long in states without local incentives or in places where there's less sunlight on photovoltaic panels.

As energy costs have risen, solar panels are finding popularity with homeowners. The payback period depends on the size of the home and the number of panels and can be lengthy, but Gellner said clients who have opted for solar panels understand that it is not a "dollars or cents decision," but a moral decision -- some people are willing to spend a lot of money if they feel they have done something right for the planet.

For a San Francisco Bay Area couple whose electric bill used to be $8,000 per year, installing solar panels made an instant difference. After installing 42 panels on the home and eight pool panels, the owners saved $6,000 per year.

"At the rate that we were going it was money out, money out, and nothing coming back in," said homeowner Susan Wilkins.

With a price tag of $30,000 to $50,000, she estimates that the panels add about $80,000 to the value of her home. The Wilkinses are looking at an eight-year payback period, but the decision to go solar was not just about the electric bill.

"It was a priority to do something positive not just for ourselves but (for) our children's future," she said. "When you put solar panels up you don't just work for yourself, you work for your community."

The eight pool panels heat their saltwater pool to 90 degrees. In addition to the energy conservation and cost savings through the panels' installation, the family also cut down on water use by installing artificial turf in the backyard, and they bought a hybrid vehicle to save on fuel costs.

Rick Wilkins, Susan's husband, offered advice to others who are studying the solar option: "Do your homework. Look at rebates offered. It's a matter of choices. What's going to matter in 10 years? Is the high-priced vehicle that you buy going to make a difference in 10 years, or will it make a difference if you spend on something lasting and enduring?"

For more information on Solar Energy potentials:

Go Solar 101

For more information on Rebates and Financial assistance for going Solar in California:

Go Solar California