The Effect of Array Tilt Angle on Energy Output

The effect of array tilt angle on solar PV energy output may be up to 20% compared to that of flat installations. A comparison of data in two US cities is given to exhibit the importance of solar PV tilt angle.

The array’s tilt is the angle in degrees from horizontal. A flat roof has 0 degree tilt and a vertical wall mount has a 90 degrees tilt angle. Whether you are installing solar panel on a flat roof or a pitched roof, the output of the solar PV system would be increased by optimizing the tilt angle.

One can ask the question, how much would the energy output change in a case where the array is tilted 30 degrees instead of 10 degrees? To answer this question; we must initially be aware of the following;

  • The Latitude

The sun, at noon time, is not always directly on top of us. The sun will be slightly to your south throughout the year, if your location is in the north of 30th latitude in the Northern Hemisphere. (That is approximately the border of Mexico and USA)

The array output is highest, if the array faces directly to the sun at all times. Since the location of the sun moves throughout the day, it is not possible to face directly to the sun unless we use 2-axis solar tracker. Yet, it’s possible to optimize the output by tilting the array at a correct angle.

  • The Shade of the Tilted Array

A tilted array on a flat roof means an obstacle for other modules in the same roof. We don’t want any shade on any modules at any time. Therefore, there should be enough clearance between the rows of the array.

Here is the answer to my previous question;
how much would the energy output change in different tilt angles?

I’ve run a simple calculation on the free software SAM (System Advisor Model) that is provided by NREL (National Renewable Energy Laboratory). The sample 3.4 kW system, consists of 16x Sanyo HIT-N215A modules and is assumed to be located in Phoenix, AZ and Portland, OR.

Array Tilt Angle
(degrees)
Phoenix, AZ (33 North)Portland, OR (45 North)
Annual Production
(kWh)
Delta
(%)
Annual Production
(kWh)
Delta
(%)
0 - flat5,72303,6240
106,178+8%4,019+9%
206,461+13%4,239+15%
306,575+15%4,355+18%
406,526+14%4,368+18%
506,310+10%4,279+16%
90 - vertical3,966-31%2,967-20%

This example shows that the solar array tilt angle of 20 to 30 degrees would significantly increase the energy output of the system. The positive effect of the array tilt angle is higher in the northern cities than in the cities closer to the equator. 

I am going to post an article about "The effect of Azimuth Angle on Solar Energy Output", which is a related topic to what we've discussed in this article.

Here are some links that I recommend for using in similar calculations:

Comments

For jobs in California you can also run the CSI EPBB Calculator to see the difference in incentives and projected production based on various tilt degrees. 


How does the Solar Advisor Model (SAM) differ from PV Watts for projecting energy production based on tilt or other design factors?  Thanks.

This is a great question.
PV Watts is a tool that is generated by NREL Renewable Resource Data Center who provides access to an extensive collection of renewable energy resource, data and maps. The tool uses the typical meteorological year data for selected location and determines the solar radiation of the PV array. It also allows users to see estimated hourly, monthly and yearly energy output datas.

SAM is a performance and economic model designed together with Department of Energy and Sandia National Labaratories. By using SAM, the user can calculate installation and operation costs, type of financing, applicable tax credits and incentives in addition to hourly, monthly and annual system performance data.

I can say that PV Watts (both version 1 and 2) is a great tool for quick estimations that could be used for quotes or project estimates. SAM is very useful on the project development phase especially for those who would like to investigate the financial part of the solar PV projects.

This is helpful, but one thing I don't really understand is that my array in the San Francisco Bay Area is tilted at 22 Degrees and my Latitude is about 37. That doesn't seem to match your table - but I was assured that 22 degrees was optimal for my grid connected installation? Can someone explain?

Thanks!

Martha,

Your optimal tilt angle is actually probably closer to 20 Degrees - your roof may be 22 deg., and the modules are just racked parallel to the roof. The reason that's considered close to optimal is because the system is designed for revenue generation under the California Solar Initiative rebate program. The CSI is heavily biased toward solar production in the Summer months (roughly May through September) when the sun is higher in the sky and electrical demand is highest in your area. The higher sun angle requires a lower module angle.

Also, is there a software tool for calculating the self-shading of an array at a given tilt angle? Thanks again for your answers...

As a rule of thumb: X >= 2*H
where X: space between rows, H: height of the tilted array
The spacing between the rows should be at least two times the height of the tilted front row.

The self-shading would change depending on the time of the day and the time of the year. You can use Google SketchUp for almost precise shading calculations.

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highprofile single leg 28.57 KB
highprofile double leg 28.13 KB
high profile tilt leg dimensions 56.7 KB
In the doucment of NREL below for the station at phoenix the monthly radiation is high in april/may but shouldn't it be high in the summer i.e. June/July/August? for a typical tilt of 20 deg.
www.nrel.gov/docs/legosti/old/5607.pdf refer (page 38)
Can anyone answer this?