Solar Irradiance Calculator - Get the full data set of solar irradiation

The amount of solar energy available for your solar array in your site depends on a number of factors:

• Your location (latitude);
• Weather;
• Season;
• Solar panels tilt angle;
• Solar panels orientation.
• Shading

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National Renewable Energy Laboratory PVWatts Calculator.

1. "How much sunlight is available for a solar array at a given location?"
2. "Direct normal solar irradiance (DSNI)."
3. "How much sunlight is available for solar panels tilted at optimum angle?"
4. "How much sunlight your roof get based on its tilt and orientation?"
5. "Tilt and Orientation Factor (TOF)."

TOF is the solar irradiation at the actual tilt and orientation divided by the irradiation at the optimum tilt and orientation (divide #4 by #3), expressed in percent.

The calculator does not consider the environment surrounding the PV system (shading and dirt).

How to Use This Calculator

Direct normal irradiation (DNI) measures the amount of solar energy received by a surface element that is always held perpendicular to the sun direction in the sky, which means it is measured by tracking the sun during the day.

If you plan to mounted the solar modules on a plane (your roof) tilted a certain angle and oriented a certain direction (azimuth) this tool will also provides you the amount of solar radiation will received based on its tilt and orientation.

This calculator provides also the optimum tilt angle in your location (OPTA), and the Global Tilted irradiation at optimum angle (GTI_opta); a measurement of the solar irradiation that would reach a solar arrays whose angle is fixed and set to the OPTA.

finally, TOF is calculated by dividing the solar irradiation available at the surface based on its tilt and orientation (GTI), by what would be available at the optimal tilt and orientation (GTI_opta). 

How to find the Azimuth of your roof

Azimuth angles of roof surface is the angle between a true north and the normal of the roof facade projected on the horizontal plane

Steps to find the Azimuth of your roof

Go to website > suncalc.org

Choose the roof facade: Identify the roof facade on which you plan to install the solar panels.

Choose the time when the sun is perpendicular to that facade: This is done by moving the yellow circle at the top of the page until the position of the sun in the sky is perpendicular to the roof facade on which the solar panels will be installed. 

The azimuth angle of the roof facade is equal to the azimuth of the sun at that time. You will find it in the section "Solar data for the selected location".

For instance, one of the roof facade of this home facing to southwest (i.e. 199° clockwise from true north), we’d enter the number 199.

Is my roof gets enough solar energy?

It depends on your roof size, and its total solar resource fraction TSRF.

The TSRF is the ratio of insolation available accounting for both shading and Tilt and Orientation Factor (TOF), compared to the total insolation available at a given location at the optimum tilt and orientation and with no shading. expressed in percent.

A higher TSRF indicates that the solar panel system is more effective in capturing solar energy, while a lower TSRF suggests that improvements in tilt, orientation, or shading mitigation may be beneficial to increase energy capture.

Effects of shading on solar panels

Solar panels work best when there is no shade casted upon them. In fact, a shade of cast on even just part of one solar panel in your solar array can potentially compromise the output of the whole system. Partial shading may be caused by trees, buildings, other objects present in the surroundings of the PV system, or even other roofs of the PV system itself.

It is also important to take into account smoke pipes on the roofs, which actually are really problematic to handle because the shade of the pipe is dynamic throughout the day and even the year.

Not only objects close to the system may cause partial shading, but also faraway hills or mountains may obstruct the direct sun irradiance falling on the modules. Hence, during the design, it is important to evaluate the impact of each possible source of shading for all months of the year. This is the task of a PV system installer and he should be able to tell you if shading will be a problem by using a range of mapping tools. 

source: Global Solar Atlas

For example, in same site in Logan Township, Pennsylvania, United States of America you can see that on December 22, which is the lowest spot traced by the sun in the sky, the sun is hidden behind the mountain from down until 9:30 am in the morning. Therefore, the modules will not receive direct irradiance in that interval time. So it is important that you understand that the shading source not always is something that you can see like a tree or a building next to you, but it can also be something very far away.

This part is just for the solar nerds out there:

Always when you have to design a PV system in a complex horizons where there are surrounding obstacles, you need to take into account shading  and calculate the solar access percentages (= 100% - shading factor) 

To calculate solar access or shading factor there exists different methods,. Traditionally, these measurements are taken on-site ( horizontal catching) using specialized tools like solar pathfinder and solmetric, but modern software allows users to build out a 3d models of the environment surrounding the PV system and simulate the shading and determine their influence using the sun’s paths.

Some software use lidar technology to accurately models the 3d height of the structure, light data is obtained by a drone or airplane which shoots out lasers to detect heights of building and trees this is helpful in modeling your 3d structure and save steps that include and build a 3d models manually, guessing what the azimuth and pitch of each facade in your roof, what the height of building and tree could be, and feed all that information into the system.

With this technology your azimuth, your pitch, your shade analysis (solar access and TSRF)  are all going to be given to you without having to travel to the site. however, it is not available in 100% of all the states.