Photovoltaic Systems

Photovoltaic systems use sunlight to generate direct current (DC) electricity. When sunlight strikes the surface of a crystallized semiconductor (typically silicon), the crystal emits an electric current. These crystals are tied together with wiring to create solar cells. A single PV or solar cell produces only a small amount of power. To produce more power, solar cells are interconnected to form panels or modules.

There are several varieties of panels available on the market. The most common is made from silicon crystals. Another newer type is called a "thin film solar array". This new technology allows solar panels to be made with less material, making the manufacturing process simpler and less expensive.

Solar panels range in output from 10 to 300 watts. They can be combined into groups called photovoltaic arrays. PV arrays can be mounted in a fixed position facing south or mounted on a tracking device that follows the sun, allowing them to capture the most sunlight over the course of the day.

A key component of a PV system is the inverter, which converts the DC power generated by the panels into the alternating current (AC) used by most home systems and appliances.

There are two basic types of PV systems - stand-alone systems or those designed to be connected to the electric power grid.

Stand-Alone or Off-Grid Systems

Stand-alone or "off grid" systems are most often found in rural settings or projects where the closest grid connection is a half-mile or more away. These systems are typically designed for cabins, RVs or other remote or portable applications. Because they are not connected to the electrical grid, off-grid systems require specialized batteries to store the electricity generated by the panels.

Grid-Connected Systems

A grid-connected photovoltaic system receives back-up power from a utility's grid when the home's electricity demand exceeds the PV system's output. When the PV system produces excess power, the utility is required to purchase the power.

The Public Utility Regulatory Policy Act of 1978 (PURPA) requires power providers to purchase excess power from grid-connected small renewable energy systems at a rate equal to what it costs the power provider to produce the power itself. Power providers generally implement this requirement through various metering arrangements.

The two most common metering arrangements are:

  • Net purchase and sale

    This arrangement requires two meters - one measures and records the amount of electricity drawn from the grid, and the other records the amount of excess electricity generated and fed back into the grid. Electricity drawn from the grid is priced at the retail rate, and the power provider purchases any excess generation at its avoided cost or wholesale rate, which can be significantly lower than the retail rate.

  • Net metering

    Under this arrangement, a single bi-directional meter records both the electricity drawn from the grid and the excess electricity the PV system feeds back into the grid. At the end of each billing period, if you've used more electricity than your system has produced, you pay retail price for the difference. If you've produced more than you've used (which is unusual), the utility company pays you for the difference at its avoided cost. The advantage of net metering is that, for systems that generate less electricity than you use, the electricity you feed back into the grid is credited at the retail price.

Before purchasing a grid-connected PV system, you should contact your electric power provider for information on their specific grid-connection requirements and metering arrangements.

Other Factors to Consider

  • Each state and community has its own set of codes and regulations that you will need to follow if you plan to add a photovoltaic system to your home. Code requirements for PV systems can vary by jurisdiction, but most are based on the National Electrical Code (NEC), which spells out requirements for designing and installing safe, reliable, code-compliant PV systems.
  • Some neighborhoods or subdivisions have covenants that restrict the types of modifications that can be made to homes, usually for aesthetic reasons. If you live in this type of neighborhood, check for any restrictions before purchasing a PV system.

Sizing Considerations

A typical residential PV system requires an area of about 8 feet by 25 feet for the solar panels, and can generate about 2 kilowatts of power. Once the direct current is converted to alternating current by the inverter, the output is reduced to about 1.7kW.

Experts recommend installing the largest system your budget permits, for two reasons. Installation costs are roughly the same regardless of the system's capacity, and the inverters, one of the more costly components, are typically sized for systems up to 5kW.

To get the most out of a PV system, first take all practical steps to reduce your home's demand for electricity. Make sure the home is well insulated, replace or update inefficient heating and cooling systems, and install energy-efficient appliances and lighting.

Tax Credits and Other Incentives

Homeowners who install PV systems may be eligible for a federal tax credit of up to 30% of the system's cost. This credit applies to systems placed in service before December 31, 2016. For details of this tax credit, consult your tax advisor or contact the Internal Revenue Service.

A number of states also offer incentives for renewable energy applications, including PV systems. Check with your state's Energy Office or your local utility to find out about any available incentives.

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