Knowing what direction a roof faces (its azimuth) is important to accurately estimate a roof-mounted PV array’s generation potential. With the use of a compass and magnetic declination, a roof’s orientation can be determined using these few simple steps.
Align the base of the compass with the edge of the roof. Most compasses will have an indicating arrow that will be perpendicular to the roof edge.
2. Rotate the compass dial so the needle is aligned to the magnetic north-and-south axis.
3. Adjust the compass dial to compensate for the local magnetic declination. You can find your magnetic declination at https://www.ngdc.noaa.gov/geomag-web/
a. For eastern declinations, the dial is rotated
counterclockwise, subtracting the declination value from the current compass reading.Or
b. For western declinations, the dial is rotated clockwise, adding the declination value to the current compass reading.
4. The compass bearing that aligns with the indicating arrow is the roof’s azimuth. The photos demonstrate finding roof orientation with an eastern declination of 8°.
Solmetric provides a free online tool that can determine your roof’s azimuth. Just enter your address and use a screen cursor to point to the edge of your roof (see bit.ly/SolmTools). To use the tool, you will need to register on Solmetric’s website.
Select “Roof Azimuth Tool” from the drop-down menu under the “Support” tab at the top of Solmetric’s home page. Log in with your username and password, and a map of the United States will appear with a text box at the top to enter the property address. Once the address has been entered, an overhead image of the property will appear for viewing. Next, click on the “Azimuth Tool” button located at the top of the satellite image just to the right of the address text box. A crosshair will appear for you to move and establish a reference point for measuring the roof azimuth. Use the crosshair to click on two points at either end of the roof edge. Once the two points have been established, the tool automatically draws a line along this roof edge indicating the orientation to true south, which is also shown in the “Azimuth (true)” text box.