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Parameters for angle calculation

user interface

SPAC user interface

The web application SPAC calculates theoretical quantities (angles and radiation) as well as practical quantities (annual electricity gain). The application window consists of four sections. 1. The Location section logically serves to select a location. The location database contains all major cities in the Northern Hemisphere with calculated climate parameters that are the basis of this web application. 2. The Roof Parameters section contains two important fields that need to be filled in in order to have a valid estimate, namely the roof slope or tilt and the roof orientation on the south. The “Solar Panels” section is the third section of the application window. It is optional, but it is actually the most important part of the application. There we choose the solar panel that we would install taking into account its power or dimensions. Under the list box for the selection of panels, there are two options: one is for the installation of panels on the roof of the house, and the other is for the desired solar power plant, for example in the yard.

First parameter

Roof orientation

The first and main parameter is the orientation of the roof. At least one side of the roof must be completely or partially facing south. This means that the roof does not have to be strictly facing south, but south-southwest or south-southeast is acceptable. If a part of the roof faces north, north-northwest, or north-northeast, its exploitation in terms of mounting solar panels will not be taken into consideration. We can easily determine if at least one part of the roof faces south with the help of a compass.

Compass

SECOND parameter

Tilt angle - slope

The second parameter that the solar calculator requires is the tilt angle or roof pitch of your roof. You will have to measure that. An ordinary protractor is sufficient for this purpose. Lean it on the slope of the roof and place the second ruler or spirit level vertically, and then read the result. There are several types of protractors—some you can buy and some you can make yourself. If we use a protractor as in the picture, it is necessary to lean the long side on the roof and place the ruler vertically. The value is read on the left side of the protractor.

protractor
A carpenter's protractor with a ruler can be used to measure the slope of the roof
Tilt
Angle measurement by simple improvisation

third parameter

Latitude

Latitude of your place 

If we have found a side that is completely or partially facing south, we can continue with further calculations. Another parameter that you need to know and that the solar calculator requires is your latitude. But don’t worry, you don’t have to look for it on maps or Google. Simply choose from the list of locations or cities in the drop-down menus. If your city is not on the list, then unfortunately you will have to look on Google for the latitude of the place you have chosen for the installation of the solar panel, which is usually your hometown. Choose from the list of locations or cities in the drop-down menus. If your city is not on the list, then unfortunately you will have to look on Google for the latitude of the place you have chosen for the installation of the solar panel, which is usually your home town.

city choose
Choice your hometown or nearest location

That would be the basis. And now the most interesting thing: how much electricity we can get from our roof or solar power plant.

roof dimensions

Solar panel section

First, select the desired solar panel from the database. The list box contains the basic data on the dimensions and power panels. Then choose one of the two options. The first is the installation of the panels on the roof. It is enough to enter the dimensions of the roof’s length and width (take care to enter the dimensions as shown in the picture). As a result, you will get, calculated according to Liu Jordan’s approximation, the surface utilization rate of the roof and the annual theoretical profit of electrical energy. Another option is the selection of the solar panel grid (again, follow the principles as shown in the picture), the number of rows, and the number of columns. It is assumed that the panels are connected regularly. In that case, apart from the expected electrical energy, we will also get output power, according to which we can further dimension the inverter.

example

Let's take a look on example

Latitude and orientation

For our example, we will take a house in the countryside. We can use Google Maps to determine the latitude. All right just point mouse on house and read latitude.  Let’s examine the location on the Google map. In our case, it is 44.117 degrees north. We will see  the orientation of the house is south east – north west. Of course, we will use the roof that faces south-east.

The first and main parameter is the orientation of the roof. At least one side of the roof must be completely or partially facing south. This means that the roof does not have to be strictly facing south, but south-southwest or south-southeast is acceptable. If a part of the roof faces north, north-northwest, or north-northeast, its exploitation in terms of mounting solar panels will not be taken into consideration. We can easily determine if at least one part of the roof faces south with the help of a compass.

As we can see from the pictures, the useful part of the roof is oriented in the south-southeast direction at an angle of 46 degrees when viewed from the south.

Roof slope

We still have to measure the slope of the roof. We will do that with a primitive do-it-yourself protractor. For this purpose, we will use the force of gravity. A protractor is a board on which a divided circle of +-90 degrees (semicircle) is glued. A hole was drilled at the coordinate beginning, and a pointer with a small weight was inserted there.

The roof slope is 44 degrees. Now we already have all three data points for the calculation. Based on them, we can calculate the radiation during the months and get approximate data on the expected profit from the installation of solar panels. So let`s do it  

Choose any option because we’ll enter the data manually anyway. Next, enter data for latitude, orientation, and roof slope.

Press “Calculation”  button

The picture above shows the ideal angles of the  panels for each months. However, our roof ( and panel) are at a fixed angle, we proceed to the next step

And now we can analyze the data. I personally use Libre office. There are three rows in the table. Received solar radiation, energy per square meter if we assume a panel utilization rate of 0.2 and the energy we get from the same panel measuring 1.65 m x 1 m (65 inch x 79 inch)

Table – data analys and chart 

excel chart

What can I conclude?

We have a fairly winterized roof with a high angle value of 44 degrees, which guarantees good usability during the winter months in early spring and late autumn. But then the days are short. During the summer, the situation is unfavorable because of this slope. I also have a relatively unfavorable orientation of the house, even at 46 degrees from the south, which further complicates the situation. All in all, I had a slight increase from January to April, a drop in the summer months, and a stabilization during the fall. The average is 260 W from the panel, which is not bad, but it can always be better.

Miroslav Mitić

Mechanical engineer and programmer for 25 years