When Are PV Systems Appropriate? People select PV systems for a variety of reasons. Some common reasons for selecting a PV system include:

  • Cost-When the cost is high for extending the utility power line or using another electricity-generating system in a remote location, a PV system is often the most cost-effective source of electricity.

  • Reliability-PV modules have no moving parts and require little maintenance compared to other electricity-generating systems.

  • Modularity-PV systems can be expanded to meet increased power requirements by adding more modules to an existing system.

  • Environment-PV systems generate electricity without polluting the environment and without creating noise.

  • Ability to combine systems-PV systems can be combined with other types of electric generators (wind, hydro, and diesel, for example) to charge batteries and provide power on demand.

PV systems are not cost-effective for all applications. The following discussion gives some general guidelines to consider when deciding whether a PV system is appropriate for your situation.

  • First, if your site is already connected to a utility grid, or within one-quarter mile of the grid, a PV system will probably not be cost-effective. Each utility company spreads the cost of its power plants and fuel costs among all its customers. Most utilities can provide electricity to consumers for about 6 cents to 14 cents per kilowatt-hour. When you install a PV system, you are essentially installing your own mini-utility system. You pay all the costs of generating the electricity you consume. Although the sun's energy is free, the PV equipment is not free. The electricity generated by PV systems at current module and balance-of-system prices can cost 20 cents to 40 cents per kilowatt-hour, depending on the installation cost and intensity and duration of the sunlight at the site.

  • Second, small PV systems are not practical for powering space-heating systems, water heaters, air-conditioners, electric stoves, or electric clothes dryers. These loads require a large amount of energy to operate, which will increase the size and cost of your PV system. Therefore, select the most energy efficient loads available. For example, if your PV system will power lights, look for the most energy efficient light bulbs. If your system will pump water for toilets and showers, look for the most water-conserving fixtures.

Is My Site Adequate for PV? A PV system designer can conduct a detailed site assessment for you. To save the dealer time (and possibly save yourself some money), you can conduct a preliminary assessment to determine whether your site has potential for a PV system. For more information, see Assessing Climate To Improve Solar Design section.

  • There are three factors to consider when determining whether your site is appropriate.
  • First, systems installed in the United States must have a southern exposure. For maximum daily power output, PV modules should be exposed to the sun for as much of the day as possible, especially during the peak sun hours of 10 a.m. to 3 p.m.

  • Second, the southern exposure must be free of obstructions such as trees, mountains, and buildings that might shade the modules. Consider both summer and winter paths of the sun, as well as the growth of trees and future construction that may cause shading problems.

  • Finally, the unobstructed southern exposure must also have appropriate terrain and sufficient space to install the PV system. A flat, grassy site is appropriate terrain, whereas a steep, rocky hillside is not.


How Does Weather Affect PV Module Output? Unlike utility power plants, which produce electricity constantly despite the time of day and year or the weather, the output of PV modules is directly related to these two factors.

Where you live will affect the number of PV modules you will need for power, because different geographic regions experience different weather patterns. Seasonal variations affect the amount of sunlight available to power a PV system.

How to Size Your PV System: To size your PV system, you must first know your energy needs, which you figure by listing all your daily loads. A load includes anything that uses electricity from your power source, such as lights, televisions, radios, or batteries. Some loads need electricity all the time, such as refrigerators, whereas others use electricity less often, such as power saws. To determine your total energy consumption, multiply the wattage of the appliance by the number of hours it is used in a day. Some appliances do not give the wattage, so you may have to calculate the wattage by multiplying the amperes times the volts. After adding the totals for each appliance, you can decide what power output you need for your PV system.

Figure 131: Photovoltaics Example


Daily Use


Total Energy
Consumption (watt-hrs.)

























Total Daily Energy Consumption

506 watt-hrs.

For the items listed above, you would need a system that produces an average daily energy output of 506 watt-hours. Obviously, different parts of the country receive varying amounts of sunlight. Because sunlight is the source of power for PV, you must determine the daily amount of sunlight in your region. Remember that PV systems are rated by peak watt, which is the amount of power produced when the module receives 1,000 watts per square meter of exposure to the sun (insolation).

Let's examine two locations: Albuquerque, New Mexico, and Pittsburgh, Pennsylvania. Albuquerque is a fairly sunny area. In Albuquerque, for each peak watt that a PV module is rated, it will produce a yearly average of 6.2 watt-hrs.* of electricity daily (*This is based on the winter average. For more precise calculations, consult month-by-month averages and use the lowest monthly average). In Pittsburgh, a cloudier area, the same module will produce an average of 2.4 watt-hrs.* of electricity daily.

If you wanted to use a PV system in Albuquerque for the appliances listed in the table, you would divide 506 watt-hrs. by 6.2, divide that by 0.8 to account for inefficiency of the batteries and, finally, multiply by 1.2 to cover anything that may have been overlooked. You find that you would need a PV system rated at 124 peak watts. If you were buying 50-watt modules, you would need three modules, because you round up to the next highest number.

506 / 6.2 = 82

82 / 0.8 = 103

103 x 1.2 = 124

124 / 50 = 3 modules

For Pittsburgh, you would divide 506 watt-hrs. by 2.4, divide by 0.8, and multiply by 1.2, which yields 317 peak watts, or seven modules at 50 watts each.

506 / 2.4 = 211

211 / 0.8 = 264

264 x 1.2 = 317

317 / 50 = 7 modules

Determining your daily energy consumption can be done through simple calculations like the example above or with the aid of sophisticated computer programs. If you are seriously considering purchasing a PV system, there are also other factors to consider. You may want to refer to other sources for more precise ways to make your calculations.

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