What Is It and How Does It Work?
Photovoltaic (PV) panels are a clean, emissions-free way to convert solar radiation into electricity. By installing PVs on your home, you can reduce or eliminate your reliance on utility-supplied electricity. PV systems can be connected to the electric grid, or they can be installed as stand-alone systems with no tie to the grid. PV systems can be implemented in a retrofit by adding them onto a roof or wall, or be integrated into the design of the building form, known as building-integrated photovoltaics, or BIPV. Although PV panels are more efficient and produce more electricity in clear, sunny weather, they can also be effective in cloudy conditions. They may not operate at maximum efficiency, but it could still be an option worth exploring if you are considering options for on-site generation of electricity.
PV panels can be mounted in a variety of arrangements. The solar cells generate the most electricity when the panels are oriented at exactly a 90-degree angle from the rays of the sun that fall on them. As a result, PV panels that are mounted on a tracking device that rotates over the course of the day and throughout the year will generate the most electricity. However, this type of device can be expensive and may not be feasible in all cases. If you are mounting fixed PV panels, there are a few guidelines to follow. The panels should be mounted facing due south, or very close to it. The angle at which the panels should be tilted depend on your latitude and the time of year when your electricity needs are greatest. If your electric loads are higher in summer, the angle should be your latitude minus 15 degrees; if your electric loads are higher in winter, the angle should be your latitude plus 15 degrees.
Why Should I Consider It?
As part of a comprehensive process of home energy use reduction, a PV system can greatly reduce your reliance on the electric grid and, in some cases, eliminate the need to draw electricity from the grid. In most cases, a PV system that provides all of your electrical needs will not be cost feasible, but if you live in an area with high electricity costs, it can be a way to save money over the long term.
What Are the Options?
The oldest form of PV panels are called crystalline, or thick crystal, panels. These panels are made of solar cells derived from silicon and look like an array of blue circles in a frame. Thick crystal panels produce approximately 10 to 12 watts per square foot of PVs in full sun.
A newer and potentially less expensive option is thin-film (amorphous) panels, which are thin layers of PV material that can be integrated into glass or metal panels. Thin-film panels produce only 4 to 5 watts per square foot of PVs in full sun, which makes them less efficient than thick crystal panels, but they are also less expensive.
In addition to the type of PV panel, you will also need to choose whether your array is connected to the electric grid or stores generated electricity on-site. Grid-connected systems are linked to the electric grid, so they have a backup power source in case demand exceeds supply. When the power generated exceeds on-site demand, the electricity is fed back into the grid. Through net metering agreements, most utility companies will pay the source of this extra electricity. Grid-connected systems have big advantages because they do not require expensive and space-intensive battery systems, and they solve the problem of electricity production not always matching up with demand. For example, if you install PV panels at a fixed angle optimized for summer sun angles, your array will produce more electricity in the summer months than during the winter when heating loads are high. If your array is grid-connected, you can feed extra electricity into the grid during the summer when your array operates at a high level of efficiency and receive compensation from the utility company that can be applied to electricity usage in the winter—without having to store any electricity generated on-site.
Stand-alone, or off-grid, PV arrays do not have any connection to the electric grid. This type of system is best implemented in remote locations that do not have access to a utility-operated electric grid, or where it would be too costly to extend the grid. This is also an effective system for more focused applications that do not draw significant amounts of power, such as outdoor lighting or irrigation equipment. If you are installing a stand-alone system in order to provide electricity for your entire home, you will also need to install some other pieces of equipment in addition to your PV panels, including a charge controller, batteries, and an inverter.
How Is It Installed?
Photovoltaic systems should be installed by experienced professionals. In your search for the right contractor you should look for someone who has broad experience installing either grid-connected or stand-alone systems. The installer should also have an electrical contractor's license. Solicit multiple bids and be sure that the bids include the maximum capacity of the array and an estimated range of annual energy production.What Are the Potential Benefits?
Photovoltaic panels require very little maintenance and create no pollution on-site, so they can serve as a clean, long-term solution to generating electricity.
Are There Health and Safety Concerns?
Due to the complexity and danger of installing electrical equipment, be sure to hire a licensed and experienced electrician to install your PV system.
How Much Does It Cost?
Although the installation of a PV system still represents a large financial investment, costs of both manufacturing and installation have steadily decreased as technology improves and manufacturers increase the scale of their operations. According to a study by Lawrence Berkeley National Laboratory, the average cost of an installed residential PV array dropped from $10.50 per watt in 1998 to $7.60 per watt in 2007. Small arrays of 2 kW or less, which is just under 20 percent of average annual electricity use in the United States, cost $9 per watt to install. There are also several different ways to receive tax credits and other incentives to install PV panels.
One way to moderate costs in new construction is to use building-integrated photovoltaics (BIPV) in multifunctional ways. If you plan to install sun shades, using PVs as shades can help to share costs. Other examples of BIPV that can share costs include solar cells integrated into the building facade to replace glass, PVs incorporated into awnings or saw-tooth roofs, and PV roofing that replaces metal or asphalt roofing panels.
What Else Should I Know?
The first step in sizing a PV system is to determine how much of your electric load you want to replace with PV-generated energy. You also need to consult with a PV installer to get an estimate of the PV module efficiency that you can expect with the type of panels that you want to use and your location and site conditions. You can use the chart below to estimate how large your system will need to be.
| Roof Area Need in Square Feet | |||||||
|---|---|---|---|---|---|---|---|
| PV Module Efficieny (%) |
PV Capacity Rating (Watts) | ||||||
| 100 | 250 | 500 | 1,000 | 2,000 | 4,000 | 10,000 | |
| 4 | 30 | 75 | 150 | 300 | 600 | 1,200 | 3,000 |
| 8 | 15 | 38 | 75 | 150 | 300 | 600 | 1,500 |
| 12 | 10 | 25 | 50 | 100 | 200 | 400 | 1,000 |
| 16 | 8 | 20 | 40 | 80 | 160 | 320 | 800 |
| For Example, to genertate 2,000 watts from a 12%-efficient system, you need 200 square feet of roof area. | |||||||
From: U.S. Department of Energy
Where Do I Start?
Reduce your overall energy consumption: In a typical home, use of PVs is not a cost-effective way to supply all of your electricity. If your first step is to take other measures to reduce your energy load, you will also be able to decrease the size of your PV array and may be able to generate all of your electricity on-site.
Solar resources: Photovoltaic systems can use direct sunlight and scattered sunlight to produce electricity, so you should not rule out PV just because you live in a cloudy climate. You should, however, examine the solar resources in your area to evaluate how efficient your investment will be. Issues to consider include the available orientation of your installed panels and the tilt angle.
Cost considerations: Consult with your PV installer to determine how much electricity your array can produce in a year, and compare this amount to your annual energy use. Compare the cost of electricity in your area to the cost of your PV system to determine when—or if—your system will pay for itself. Investigate local incentives and grant programs.
Where Can I Get More Information?
