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Designing a building or roof for solar power.

These days more and more people are thinking of installing solar power. Several reasons are driving this, climate change, load shedding, power failures and rising electricity prices.

If you are designing or a new building and are thinking of installing solar power, you probably have a few questions regarding the orientation and tilt of the solar panels and how that affects the energy production of a solar power system.

Designing a building and its roof correctly for solar from the start is very important to ensure that solar power is viable and enough for a specific building. Having control of the building designing from the start also enables you to make provision for the location of the solar equipment, So, let’s talk about solar panel orientation, incline and energy production.

Understanding solar system power.

The size of solar power systems is measured in kWp (kilo Watt peak). That is the combined amount of power of all the solar panels installed on the roof.

The typical solar panel size in the year 2021 is 365 Watts.

So, three of these panels installed on your roof, will give you 3 x 365 W = 1095 Watts of power, or rounded up 1100 Wp, and converted to kilo Watt it will be 1.1 kWp.

Three panels installed on a roof like this, would be one kilowatt peak of solar power installed.

To see the datasheet of a typical solar panel, click here.

one kilowatt peak solar power

Understanding solar system energy yield.

If we take one kilowatt of solar power, we can from historic records calculate very accurately how much energy it will produce for us over a period of one year. The energy produced by a specific size solar system is dependent on the following factors.

  • Location of the system in the world
  • The orientation of the solar panels
  • The tilt of the solar panels

For example, one kilowatt of solar power with the following details:

  • Location: Johannesburg
  • Orientation: North
  • Tilt: 20 degrees

The total energy production per year will be: 1731 kWh (kilowatt-hours) of energy.

The following specific solar energy production map will give you an idea of the expected energy yield of one kilowatt of solar in different areas of South-Africa:

specific solar energy production map

Orientation of PV

What is the best orientation for a solar array, for the most solar production?

The ideal orientation in the Southern hemisphere is North facing. However, East and West facing roofs can also be used, but it does have slightly less production than a North facing roof.


If the orientation varied what is the reduction/deviation of the efficiency

Based on a solar system installed in Johannesburg, at a 25 degrees incline, the following table illustrates the different yearly energy production figures.

 Orientation in DegreesProduction kWh/kWp% difference from max.
North01 730 
North East451 6505%
East901 47415%
West2701 51612%
North West3151 6733%

Inclination of Solar panels

One can also use the words tilt or angle of the solar panels. I will be using the word inclination.

What is the ideal inclination, for the maximum energy production?

Again, this varies according to where in the world the solar system is installed. At a fixed tilt the best inclination is normally the same as the latitude of the location.

From the following map, we can see for example that Johannesburg is at a latitude of 26 degrees. So, if panels are installed at that inclination, that will give the maximum energy production.

solar latitude map of South Africa

What is the effect of different inclination of solar panels on energy production?

Inclination has much less of an effect than orientation and thus is not so important. The following table shows the specific energy production of a North facing array installed in Johannesburg.

Inclination in DegreesProduction kWh/kWp/y% difference from max.
01 6404%
101 6583%
151 6842%
201 7061%
251 7150%
301 7140%
351 7021%
401 6802%
451 6494%

If the PV is horizontal what is the reduction of the efficiency

From the table above one can see that the reduction in energy production is about 4%.

What is the ideal angle of PV to ensure no salt + dust build-up?

10 degrees or more will ensure that dust runs off the panel and does not build up. We had a system installed on a flat roof with a 5-degree incline in Midrand. The panels got really soiled in the winter and had about 30% less production just because of the soiling.

Choosing the best orientation and incline.

The best orientation incline of a solar array is flush with the roof where possible, even if that means a bit less production than at perfectly North facing installation at a 25 degrees incline. The bit of extra production is normally not worth the extra cost of the mounting structure. Secondly, if the panels are installed at an incline, there is a risk that they can be blown away. Thirdly it sometimes does not look good if not installed flush with the roof. So, if one, for example, has 15% less production because the panels are installed on a West or East facing roof, rather than a North facing roof, but one can fit in more than 15% more panels then overall one would have more energy production in an East-West installation.

In the following example, the installation would have been better if it was just installed flat on the East and West roof instead of going through the expense and effort of trying to make it North facing.

East West Solar Installation

In this example, a slight incline of say ten degrees would have been better and would prevent it from blowing away with the first strong winds.

Solar system energy production calculators

For a quick simple calculator, you can use the following tool from IBC solar. This calculator will calculate your expecte yeild based on

  • Location
  • Inclination
  • Orientation

Click here to open the IBC Solar Calculator

In conclusion.

By contacting us we can calculate very accurately what the energy production would be for your current or planned building. We can plan solar systems from Google Maps or from AutoCAD drawings.

Click here to contact us

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