After you have assessed the appropriateness of your site, you need to determine how much electricity your PV system must generate. This depends on how much electricity your loads require. Again, your dealer can help you with sizing a system that will meet your needs. The sidebar illustrates the steps involved in sizing a 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.

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. 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 (see Source List) for more precise ways to make your calculations.

*This is based on the winter average. For more precise calculations, consult month-by-month averages and use the lowest monthly average.