Solar Azimuth and Altitude Angle
The optimum solar panel angle and the solar azimuth angle are essential specifications for a solar photovoltaic (PV) system. To maximize solar power production, solar panels must be oriented as close as possible to directly facing the sun. The position of the sun varies horizontally as it traverses East in the morning to West at night. And it also differs vertically as it is higher in the sky during midday and lowers in the sky during mornings and evenings.
If in the northern hemisphere, the most optimal solar panel orientation is always south facing. For panels in the southern hemisphere, the optimal orientation is always north facing.
The solar azimuth angle refers to the horizontal aspect of the sun’s position and is measured in degrees where, if in the northern hemisphere, 180° is true South, 90° is East, 270° is West, and 0° is North. Please refer to the figure below.
Sun charts are useful for understanding the sun’s position at any time of the day during any time of the year. For example, shown below is the sun chart for zip code: 94043. See Sun Charts.
The vertical axis represents the solar elevation or the altitude angle, and the horizontal axis represents the solar azimuth. Shown below are examples of how to read the graph:
~ At 12 pm on July 7th, the sun is facing directly South (180°), and the altitude angle is about 75°
~ At 5 pm on February 14th, the sun is oriented at 240° (Southwest), and the altitude angle is about 15°
~ At 11 am on April 15th, the sun is oriented at 150° (Southeast), and the altitude angle is about 60°
As you can see, the solar elevation and irradiance is highest at noon from a perspective pointing directly South (180°). It should be noted that the closer your panels point directly south, the more energy your system will absorb. But that is without considering environmental factors (i.e., clouds, dirt, weather, trees, etc.).
Array Tilt Angle
While the solar azimuth angle is critical, so is the altitude angle, which dictates the array tilt angle, or the solar panel angle. The array tilt angle should be such that the panels are oriented perpendicular to the sun’s rays. To achieve this, the panels should form a right triangle with respect to the sun’s rays, as shown below.
Thus, to calculate the optimum array tilt angle, you must subtract the altitude angle from 90°. For example:
If the altitude angle is 65°, then the optimum array tilt angle is 25°:
90°- 65° = 25° = Array Tilt Angle
When the array is perpendicular to the sun, solar modules collect the most energy.
What’s the Best Array Fixed-Tilt Angle?
For most residential systems, the array tilt angle is fixed, and thus, it can be beneficial to orient the array at a certain angle to absorb the most energy when needed. To get the best average solar irradiance on a solar array, the solar array should be tilted at an angle equal to the latitude of its location.
For harvesting the most energy in the winter, the array tilt angle should generally be oriented about 15° above the latitude angle of the array location since the sun is lower in the sky during the winter. For example, if the array is located at 45° Northern latitude, the array tilt angle should be at 60°.
In contrast, to harvest the most energy in the summer, the array tilt angle should generally be oriented about 15° below the latitude angle of the location since the sun is higher in the sky during the summer. For example, if the array is located at 35° Northern latitude, the array tilt angle should be 20°.
As you can see, it could make sense to manipulate the tilt of an array to optimize its power absorption. To measure the tilt, you can use an inclinometer.
On roofs, tilt can be added through tilt legs, which elevate the rails to affect the panels’ tilt, as shown below. Brands like UNIRAC, Ironridge, and Ecolibrium, offer tilt options for roof-mount, ballasted, and ground-mount systems. Shown below is a roof-mount set up with tilt legs added to the rails to adjust the tilt of the array.
For ground mount, the tilt is affected by the design of the ground-mount and the supporting pipes.
For a ballasted system, the tilt is integrated into the bases’ design, as shown below.
Is it Worth it to Add Tilt if My Roof is Already Tilted?
This depends on the tilt of your roof and the size of your system. Websites like PVwatts.nrel.gov can give you a good idea if adding tilt to your array from an already tilted roof makes sense.
For example, let's look at Example 1, a house in Mountain View, CA, with a tilt of 20°, a typical roof pitch for most residential homes. If this house has a 4 kW array size with an array pointing directly South, it is projected to produce on average, 6,384 kWh/Year. However, the latitude of Mountain View, CA, is at 37.45° Northern latitude. Thus, the highest average irradiance would theoretically be absorbed if the array was tilted at 37.45°, pointing directly South.
In Example 2, the projected production of the 4kW array is 6,505 kWh/Year, which only amounts to about a 2% increase from Example 1. Therefore, although you may be maximizing your array’s power absorption, changing the tilt (from a fixed-pitched roof) might only amount to a small percentage of added power. In contrast, with larger systems (i.e., 1MW+), this small percentage will to a significant amount of energy, which is why sun trackers are typically used for these applications to maximize power absorption.
The tilt angle and azimuth orientation of an array are crucial factors for maximizing power production. However, at a certain point, incremental changes to these factors account for a small percentage in the change of power production. Thus, making dramatic provisions to an array or a household (roof) might not be worth the cost and effort, depending on the owner’s preference.