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Selecting the Size of Your Solar PV System

Whether your goal is to generate your own clean energy, increase your home’s appraisal value, save money on your electric bill, or all of the above—investing in a small-scale solar electric system is a wise decision. A small solar electric system—or distributed generation (DG)—can produce reliable, emission-free energy for your home or business. However, it is important to make sure that your solar photovoltaic (PV) system is correctly sized, sited, installed and maintained, in order to maximize your energy performance.

Once you make the commitment to go solar, the next step is determining how big your solar PV system must be to meet your home electricity needs. “There’s no one-size-fits-all,” says John Ogrodny, general manager of Photon Works, an installer and manufacturer of photovoltaic (PV) systems.

solar installers installing solar panels

Start by reviewing your electricity bills over the past year to get an idea of your typical electricity usage measured in kilowatt hours. For example in 2011 the average American household used 11,280 kilowatt hours (kWh) of electricity per year, reports the U.S. Energy Information Administration (EIA). Many utility companies also offer complimentary energy audits, as this can provide greater insight into your family’s energy use habits.

After you know how much electricity your household utilizes, you have to decide the amount of energy you want to offset. While 100 percent seems appealing, it may not be financially feasible or practical due to space constraints. As a general rule, 100 square feet of solar panels will generate 1 kilowatt (kW) of electricity—but only when the sun is high and perpendicular to the panels. The amount of power generated by a solar system at a particular site depends on how much of the sun's energy reaches it. Most residential solar electric systems require between 50 square feet (for small "starter" systems) and 1,000 square feet. Commercial systems may require even more surface area or rooftop space.

To further complicate matters, certain solar PV modules generate more electricity per square foot than others. Monocrystalline (black, single cell) panels have traditionally boasted the highest peak efficiency, although polycrystalline (blue, multiple cell) panels have caught up quickly and outperform single-cell panels on cloudy days. Thin film (amorphous) panels are more flexible, but less efficient than crystalline panels and have declined in popularity since 2009. Scientists have also been experimenting with nanotechnology to increase solar cell efficiency while driving down costs.

Besides your energy usage, the size of your solar PV system will also depend on your geographic location. A 1.5kW system may power the entire home of an Arizona family, where the same system may only reduce a Midwest family’s electricity bill by 15 percent. If your location limits the physical size of your system, you may need to install rooftop system that utilizes higher-efficiency PV panels or think outside the rooftop with solar awnings or a solar carport.

When seeking bids, it's a good idea to gather multiple quotes from different solar companies. Listen to the different solar PV options that meet your budget needs and ask plenty of questions regarding panel efficiency, output and payback period. “Most solar electric systems last 30 years and pay for themselves within four to five years after tax credits and rebates,” reports the National Renewable Energy Laboratory (NREL).

Many people assume that the bigger their solar PV system, the more savings they’ll generate. While the benefits are undoubtedly greater with more solar panels, other factors may be considered before covering every inch of your roof in photovoltaics. “To make the most of your solar power, the key is to implement simple energy efficiency strategies,” advises Element Energy. For example, rather than investing in a massive rooftop solar array, homeowners could integrate energy-efficient heaters, solar hot water systems or design features such as strategically-placed vents and insulation.

To qualify for net metering (the billing agreement where you produce your own electricity and sell the excess power back to the utility company) your PV system must be under a certain generating capacity. Net metering requirements vary by state, but the typical size cap is 10kW for residential systems.

But economies of scale can certainly decrease the cost-per-kilowatt-hour of your solar PV system, as you increase in size. “Many inverters are sized for systems up to 5 kilowatts, and if your PV array is smaller (say 3 kilowatts), you may still end up buying the same inverter,” reports the Renewable Energy Resource Center. Labor costs may also be comparable for both large and small systems, and your solar installer may even quote a better price to install a 2kW system up front if you’re considering future expansion—as multiple site visits are more costly.

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