Solar geometry

The web application SPAC calculates theoretical quantities (angles and radiation) as well as practical quantities (annual electricity and profit gain).

The application window consists of 3 sections. The main section on top requires the 3 most important data entry fields:

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 included in the list, select the nearest city or search Google for the latitude of your location.

Choice your hometown or nearest location. That would be the basis for calculating how much electricity we can get from our roof.

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. Place it on the roof’s slope, align the second ruler or spirit level vertically, and then read the result.

Some protractors are homemade, while others are sold. If we use a protractor as shown in the picture, we must lean the long side against the roof and place the ruler vertically. Read the value on the left side of the protractor.

At least one side of the roof must be completely or partially facing south (south-southwest or south-southeast is acceptable). We will not consider mounting panels on roof sections facing north, north-northwest, or north-northeast. You can easily determine orientation using a compass.

Using compass for determining wall/roof orientation.

We use standard notation: 0° for South, 90° for East, -90° for West.

First, select the desired solar panel from the database. The list box contains basic data on dimensions and power output. Then choose one of two layout options:

1. Direct roof installation: Input the roof length and width. The Liu-Jordan model will compute the surface utilization rate and theoretical annual yield.
2. Solar panel grid: Specify the number of rows and columns. This yields expected total output power to assist in inverter sizing.

Latitude and orientation

For this example, we take a rural house. Coordinates on Google Maps show 44.117° N. The house aligns along a southeast-northwest axis, so we select the southeast-facing roof section.

Measuring pitch with a DIY weighted protractor yields a roof slope of 44°.

With latitude, orientation, and slope established, we can project monthly irradiance and panel output (e.g., standard 1.65m × 1.0m module at 20% efficiency).

A steep 44° pitch optimizes winter and shoulder-season gain, though it slightly reduces mid-summer potential. Combined with a 46° offset from due south, production peaks steadily from spring through autumn. Average expected panel output resolves to roughly 260 W.