Photovoltaic Systems “PV” or solar electric: Compared to solar hot water, photovoltaic (pronounced: foh-toh-vol-tay-ik) is a relatively new technology. The first photovoltaic effect was discovered by Edmund Becquerel, a 19-year old French experimental physicist in 1839. Albert Einstein received a Nobel Prize in 1923 for explaining the photovoltaic effect. But not until Bell Labs in 1954 did solar PV finally reach a level where its power began to be useful for commercial purposes, such as Western Electric’s dollar bill changer in 1955.

Unlike a solar hot water system, which is essentially a plumbing device, PV uses semi-conductors and sunlight to make electricity. The more solar modules a PV system or array has, the more electricity will be generated. DC electricity can be “inverted” into alternating current (AC), so it can be useable power for a home or business, which can off-set or even eliminate the electric bill.

PV systems to power buildings fall into four general categories:

• Grid-Interconnected or “Grid-Tied” PV systems are the most popular and use special inverters to allow electricity to flow safely back into the electric grid. When solar power is generated, this power is typically first used by the building, and then surplus electricity can actually flow back into the grid, giving full retail credit per kilowatt-hour from your utility provider. Since there are no batteries, these systems cannot stored energy and are designed to shut down if the grid is down for safety reasons (mainly to protect utility line workers).
• Grid-Interconnected with Battery Back-up systems offer customers continued power when the grid goes down, while still being connected to the grid for seamless power. Newer systems also accept other power sources, in addition to PV, such as wind or even traditional gas-powered generators to provide power and/or charge the battery at night and/or if the grid is not available.
• “Off-Grid” PV systems are used when a completely independent or “stand alone” system is needed. Since no grid power is used, the system must be carefully designed based on power usage, peak demand and seasonal solar variations. Batteries are typically used to provide power at night, in low sun or high electric demand conditions. These systems are ideal for remote locations where no utilities exist.
• Utility-Scale PV systems, sometimes called “solar farms” provide power for regional users by (or in cooperation) with electric utility providers.

Grid-tied systems may be metered by two different methods:

Net metering is the practice of using a single utility meter that “nets out” both what is “drawn” from the grid and what is “returned” or fed back to the grid. When a PV system generates power beyond what the building is consuming, this surplus power is fed back into the utility grid, making the electric meter actually spin backwards. If you generate more electricity than you consume at the end of the month, the customer will receive full retail credit (and possibly cash) from the utility provider per their policy.

Dual metering configurations use two separate meters. One meter tracks the total energy consumed by the building and the other meter tracks total energy produced by the solar and fed back into the grid. Because this method accurately meters both the total energy consumed and solar energy produced, different billing rates can be applied by the utility. This metering method is used for Feed-In-Tariff (FIT) programs where customers can be paid for solar power generated, typically at a higher rate than the conventional electricity purchased.

Regardless of PV system or metering, most homeowners will install a solar hot water system along with the PV system. Why both? Because a solar hot water system is significantly more cost-effective and requires a fraction of the roof space to create the equivalent amount of energy to heat water. This will also allow the PV system to satisfy a higher proportion of household electric demand, making the PV system even more cost-effective.

PV systems are rated by “standard test conditions” (STC) wattage during peak sun intensity. Most residential grid-tied PV systems will typically range from 2 kilowatts to 8 kilowatts. The total energy per year it generates will vary depending on the part of the country in which it is located and other factors related to design and installation. In Florida, for example, a 5 kilowatt PV system will generate about 700 kilowatt-hours per month of clean, renewable energy on average, based on a one-year period. At 15 cents per kilowatt-hour, this will offset $1,260.00 of electricity. As for carbon dioxide, the EPA reports that each kilowatt-hour of electricity produced from a coal creates 2.3 lbs. of carbon dioxide, so this 5 kilowatt residential PV system in Florida will also offset about 19,320 lbs. (9.7 tons) of carbon dioxide per year.