Whenever the sun is out, your home's roof is exposed to solar energy. Why not harness that energy, reduce your electric bill and limit the amount of carbon dioxide you add to the atmosphere each day? Depending on where you live, your roof's orientation, and the load your roof can bear, mounting photovoltaic cells (PVs) on your roof could be a valuable investment in increasing your household's energy independence.

* What are PVs and how do they work?

Certain materials like gallium arsenide, crystalline and amorphous silicon, and copper indium diselenide produce electricity when they absorb light. This is called the photoelectric effect. Solar cells are typically made of a thin layer of such photosensitive material. Light hitting the solar cell raises the energy level of the electrons within. At an elevated energy state, the electrons are able to escape from their molecules and create an electric current.

* How can you tell if your roof is a good candidate for solar cell installation?

Solar cells work best when exposed to direct sunlight. Ideally, you could put a panel of cells on a motorized mount that follows the sun's path through the sky, but that's usually not practical for a roof. The next best thing is a roof mounting where the pitch has an unobstructed southern exposure. Depending on your latitude and the angle of your roof's pitch, your PV system installer may suggest optimizing the angle of your PV modules with an elevated mounting rack so that they can catch more light.

You also need to be cognizant of how much weight your roof can bear. If your roof already has two layers of shingles, you probably shouldn't add solar panels to the load.

An alternative to solar panels is a relatively new product, "solar shingles". These are shingles into which PV cells have been incorporated. If your roof needs to be replaced anyhow, solar shingles offer the opportunity to kill two birds with one stone.

Finally, PV cells lose efficiency as the temperature rises. In warm climates, you may be better off exploring alternatives like wind-power. In places like Florida and California, public utilities are bringing solar thermal electric power plants online. Such systems, though primarily feasible only for installations much larger than the single-family home, actually turn out to be more efficient and cheaper than PV systems when it's hot.

* What components make up a roof-mounted PV system?

PV cells output direct current, and can be connected in series to increase the output voltage. Though there are specialized appliances available that run on DC, most PV systems have the cells send electricity through an inverter. This converts the solar cells' DC to standard household alternating current. The output from the inverter is then integrated into the house's electric system. Smaller systems mostly just supplement power from the electric company, reducing your overall bill.

If you install a larger system and make your home very energy efficient, you may have often had a surplus of electricity. You can either store this for later in on-site batteries, or feed it back to the electric grid. If you feed your surplus back to the grid, the electric company may actually pay you. This is called "net metering", and is the law in more than thirty-five states. It's a good deal for you, and it's a good deal for the electric company because your PV system is most likely to produce surplus electricity exactly when peak demand occurs. Your surplus-producing PV system could actually help avert power outages.

* How much does a PV system cost?

The initial cost of a roof-mounted solar array can range from $6,000 for a small supplementary system to $30,000 or more for a large "off-the-grid" system. The capital cost of such a home-based PV system is rather abstract, though. It would be better to compare what you are currently paying per kilowatt-hour to an amortized per kilowatt-hour cost for a PV system. According to the U.S. Department of Energy, that cost is about $0.25 per kilowatt-hour. In most places, this is more than what the utilities charge.

That's the bad news.

The good news is that there are many state-sponsored incentive programs for solar energy. Take advantage of them, and you could break even. For information about these programs, you should consult the National Database of State Incentives for Renewable Energy (DSIRE), prepared by the North Carolina Solar Center. The database covers incentive programs meant to promote the development of renewable energy.

In addition, the cost of the electricity your PV system produces is insulated from deregulation, rate-hikes, and inflation --- that amortized twenty-five cents per kilowatt-hour cost will still be twenty-five cents thirty years from now. Who knows what your local electric rate will be?

And there's more good news. By going solar, you are reducing the amount of coal and natural gas burned at power plants, you are reducing the amount of carbon dioxide we pump into the atmosphere, and you are asserting your energy independence.

Putting solar cells on your roof is an idea whose time has come. Home-produced solar electricity is feasible, makes sense, and is within the reach of most homeowners.