Site Assessment For Solar Energy Use

How do you assess a site for solar energy use?

Opting to use residential solar energy isn't a matter of just buying some panels and mounting them on the roof. Not all regions are geographically suited for solar energy collection. The first step in integrating solar energy in your home is simply finding out what you can expect to harvest at your specific location.

Solar Measurement Basics

Solar site assessments and subsequent system design (in terms of capacity) are based on calculations of peak sun hours. There are two things you need to understand about that term:

- A peak sun hour is one hour of solar energy on a clear summer day falling on one-square meter at the correct angle to the sun.

- In ideal conditions, one peak sun hour equals 1 kWh per square meter.

Two additional measurements with which you will need to be at least nominally conversant are:

- irradiance: The rate of solar radiation falling in a specific area at a moment in time. This is measured as kW/m2 or kilowatts per square meter.

- irradiation: Popularly this is often expressed as "radiation," because that term is more easily understood. This is simply the amount of solar energy falling on a site over time. It is measured as kWh / m2 / day. (Read that as kilowatt hours per square meter per day.)

In areas prime for solar collection, this will be 5 to 6 hours a day. When the sun is especially high or low in the sky, casting long rays that hit the array at low angles, solar power collection will be negligible.

Correct Angle to the Sun

The solar cells on photovoltaic panels are etched on to the surface as small pyramids to collect light falling at different angles and the surface of the panel is coated with a light diffusing material, but you can still only expect a window of optimal solar collection two to three hours on either side of high noon.

In photovoltaic systems, the optimal angle of the panels depends on the latitude of the site. In general, a solar array is placed at the site's latitude plus or minus 15 degrees depending on the season of the year.

It isn't unusual, however, for panels to be placed at an angle equal to the site's latitude with no calculation for seasonal bias. In the northern hemisphere, panels face solar south; in the southern hemisphere, solar north.

Average Annual Peak Sun Hours

There are numerous maps available online that show the average annual peak sun hours by region in the United States as well as globally. A good place to start is the "Basics" section at (Map links are at the bottom of the page.)

Remember, however, that what these maps convey is the amount of solar irradiation that can be converted to electricity by a fixed solar array facing true south at the optimal angle of tilt -- in other words a more or less "perfect" scenario.

Calculate Available Roof Space

Your home's roof space will also need to be evaluated for placement of the solar panels. In terms of physical space, and depending upon the equipment selected, you can roughly estimate 85 square feet of roof per kilowatt. (The average size in square footage of a roof on an American home is 2,400 square feet.)

As a general assumption, 3.6 kW photovoltaic solar array can supply half the annual energy consumption of an American home, or about 5,000 kWh per year. Rounding up to 4 kW for calculation purposes, you'd need approximately 340 square feet of roof space (with the correct angle and orientation to the sun) to install the panels.

Assuming, then, that your site supplies adequate hours of peak sun and that your roof can accommodate panel placement, what exactly can you expect in terms of power output of your solar power system?

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3 Responses to “Site Assessment For Solar Energy Use”

  1. How Much Power Can A Home Solar System Produce? Says:

    […] Site Assessment For Solar Energy Use […]

  2. Cost To Install A Home Solar Array Says:

    […] come into play, it is extremely difficult to say what a home solar power system costs. Individual solar site assessments and equipment projections by capacity, paired with personal energy usage data are required to […]

  3. Solar Energy Ontario Says:

    You’re right, there are many amazing applications in which solar panels can be used. There are also many government programs where homeowners can generate good income!

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