Basic Steps to Plan a Photovoltaic Solar System Installation

1° Calculate Potential Solar Radiation Power

If you are planning to install a Photovoltaic Solar Energy System, the first thing you need to do is figure out how much solar energy you can expect your panels to absorb, based on the average daily solar radiation for your area. For most parts of the United States, an average of 4-5 kWh/m²/day can be achieved, though in some regions it is possible to achieve up to 8.

To find the output for your area, please refer to this map put out by the National Renewable Energy Laboratory (NREL) Resource Assessment Program. In the delineation kWh/m²/day, the kWh stands for kilowatt hour, with “m” being the area of the absorber, or in this case, the photovoltaic cell, put into square meters. Solar panels, on average, work at about 70% efficiency.

Therefore, if you live in an area with an average solar radiation of 4.5 kWh/m²/day, and you have a total surface area of 2m² of .70 photovoltaic cells, you can expect to generate 6.3 kWh/day or about 180/month.

 2° Determine Average Energy Usage

Once you know how much energy you can expect to get from the sun, you can determine about how much energy you will need based on your current monthly energy consumption. If you don’t have copies of your last 12 bills, you can view the state by state averages, as published by the Energy Information Administration here. According to the chart, a consumer who lives in New Jersey, for instance, will use an average of about 730 kWh/month in electricity, while one in Alabama may use as much as 1300 kWh of energy per month.

3° Plan a Photovoltaic System Placement

A good rule of thumb to use when determining the correct tilt of solar panels is to find out the approximate latitude of your area and set the panels at an angle equal to that latitude. This should maximize the amount of solar radiation absorbed all year.

If the southward facing roof on which the panels will be installed is slanted beyond the angle of your area’s latitude, then the panels should be lifted at the bottom edges, in order to achieve the best radiation angle, whenever possible. For flat roofs, any of the four sides can be elevated to match the latitude angle.

4° Calculate Number of Panels Needed

Now that you know how much solar radiation is available and how much of it can actually be utilized by a solar panel, you can calculate how many photovoltaic solar panels you will need in order to replace your current source of electricity.

You will need to find out the exact size of each cell in square meters, as well, which can probably be found on the website of, or in literature from, your manufacturer of choice. If we assume an average energy usage of 1000 kWh per month, we will calculate solar radiation x surface area x 70% x average days per month or kWh/day x m² x 0.7 x 365/12. We will then divide that number into 1000.

For example, in an area with solar radiation of 5 kWh/m²/day, with panels that are 1m² each, the equation would be 5 x 1 x 0.7 x 30.42=106.5 kWh per panel. 1000/106.5=9.4, so you would need 9.4m² of panels to completely eliminate your electricity bill. If you go ahead and use a full 10m², you should have the opportunity to sell back energy to the electric company.

5° Calculate Upfront Cost vs. Long-term Savings

For nearly everyone, the upfront cost of a photovoltaic solar system will be a major factor. However, with the yearly utility savings and the government incentives, it can be well worth the initial investment. A lot will depend on the size of the system you install.

If you opt for a small system that generates 106.5 kWh/month, for example, and your electricity company charges 8.2 cents per kWh, you would save about $105/year, before any government incentives. If that system cost $1000 and the government rebate was $800, it would take you about 2 years to pay off the system and begin truly saving. In areas where the kWh cost is higher, the savings and pay-off period could be significantly reduced, allowing you to pay off the system in a year or less.