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I have noticed that, in my country India, most of the solar panels are tilted southward at an angle of ${45}^{\circ} .$ Even on buildings with inverted V-shaped roofs, solar panels are still oriented southward on both the sides of roof.

Research

Many sites suggests that the tilt aids in self-cleaning also another site stated that tilt depends on factor like latitude

My questions:

  1. Why are solar panels tilted southward?

  2. How is latitude of the location of a solar panel relevant in increasing efficiency?

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    $\begingroup$ Separate from all of the real answers: I've seen a lot of panels that are simply installed wrong. Where I live there are a lot of secondary traffic-lights that are stand-alone solar-powered LED units; the installers just point the panel whichever way they like, and I guess it's good enough... $\endgroup$
    – ShapeOfMatter
    Commented May 12, 2019 at 16:11
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    $\begingroup$ @ShapeOfMatter: How high were the buildings around the solar powered LED units? It could be better to get diffuse irradiance from the northern part of the sky than to get nothing at all because there's a high building right to the south of the solar panel. $\endgroup$
    – Eric Duminil
    Commented May 13, 2019 at 9:13
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    $\begingroup$ In the Southern hemisphere panels would be tilted Northwards. On the equator flat would be the optimum geometry, but self-cleaning requires an angle to cause rain (and dirt) to run off the panel. $\endgroup$
    – nigel222
    Commented May 14, 2019 at 12:40

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First, not every solar panel in India is oriented towards the south or tilted at 45°. One of the world's largest photovoltaic power stations is installed in Kamuthi (9.3°N, Southern India), with pv modules tilted at 8°.

Azimuth

Panels are usually oriented towards the south in the northern hemisphere because the sun mostly is in the southern part of the sky. The sun sometimes is in the northern part of the sky, e.g. during sunrise and sunset in spring and in summer. It only happens when the sun is relatively low so it doesn't have a huge influence on the total yield.

Here's a sun-path diagram for New Delhi (28.6°N, Northern India):

enter image description here

When solar panels are installed on buildings, they sometimes have to be integrated directly in the roof, so the orientation will be dictated by the architecture.

enter image description here

Depending on whether the electricity will be used on location, stored in batteries or sold to the grid, it might be interesting to produce less electricity per year but to produce it when it is most useful, e.g. during the afternoon for air conditioning. In that case, solar panels could be turned towards the west.

Tilt

Finding the best tilt angle is a compromise :

  • too low and the panels won't be cleaned by rain.
  • too low and the panels won't produce much in winter.
  • too high and the panels won't produce much in summer. This can be desired for solar thermal collectors, because boiling water could damage the pumps.
  • too high and the rows will shadow each other.
  • too high and the panels and mount will have to withstand higher forces in windy conditions.
  • too high and the rows will have to be wider apart. Since pv modules are getting cheaper and cheaper, the current trend is to put the modules almost flat, and as close to each other as possible. This way, a larger capacity can be installed for a given roof size.

45° tilt seems to be too high in India for photovoltaic panels:

enter image description here

It could be about right for hot water production:

enter image description here

Finally, this angle might have been dictated by architectural choices.

Here's an average irradiance vs tilt diagram for New-Delhi (28.6°N):

enter image description here

and for Kamuthi (9.3°N):

enter image description here

In both case, the curves are pretty flat around the maximum, so the tilt angle could be chosen to be 20° or 25° in New-Delhi in order to avoid shadows. It shouldn't be much flatter than 10° in Kamuthi in order to avoid soiling.

Azimuth & Tilt

North India

Here are contour lines for yearly insolation vs orientation in New-Delhi:

enter image description here

Unsurprisingly, the orientation with the highest yearly yield is towards the South with a tilt between 25° and 30°, for an insolation of almost $2150\mathrm{\frac{kWh}{m².a}}$.

South India

enter image description here

Kamuthi is so close to the equator that the azimuth doesn't matter much, as long as the tilt angle is low. If the tilt angle is higher (e.g. around 60° for solar thermal collectors), it's actually better to orient the panel towards the East or West than towards the South.

Sources

  • Every diagram has been generated with Ruby + INSEL + Gnuplot.
  • Monthly irradiance data has been downloaded from PVGIS.
  • Hourly values have been generated with Gordon-Reddy.
  • Hay & Davies diffuse sky model has been used to calculate global irradiance on tilted planes.
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  • $\begingroup$ As you say, the curves are pretty flat, so it might be a case of "what is easier to build". If you look at the New Delhi curve, at 45° you still get > 95% efficiency. It's a lot easier to build something for 45° than for an arbitrary angle. $\endgroup$
    – semi-extrinsic
    Commented May 13, 2019 at 7:56
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    $\begingroup$ @semi-extrinsic I'm not sure it's easier to build a mount with 45° than with a lower angle. You need more material, the panels are more visible, they shadow each others more and they have to withstand stronger forces when it's windy. The only 45° tilted pv panels I've ever seen (even in northern Europe) were integrated in a 45° roof. $\endgroup$
    – Eric Duminil
    Commented May 13, 2019 at 9:05
  • $\begingroup$ Is the average irradiance vs tilt observed or theoretical, and if theoretical, what does it take into account. It makes a difference, because if it is based on the assumption that the atmosphere absorbs no radiation you will get one result. If you assume that the atmosphere is made of a material with a uniform absorbance, you will get another result as the light from the lower sun in morning and evening has to travel through more atmosphere. If you take cloud cover into account then this will have different effects at different times of day and year. $\endgroup$
    – David Robinson
    Commented May 14, 2019 at 10:12
  • $\begingroup$ This answer seems to assume that all energy is equally valuable so you should maximize total energy collected. However, energy is more valuable when it is more scarce, and when demand is highest, so these need to be taken into account in the function being maximized. In the extreme case, if you try to get all your energy from solar, then you will have to erect enough panels that you have far more than you need in the middle of a summer day when there is no cloud, and so the best design is the one that produces the most energy when the sun is low. $\endgroup$
    – David Robinson
    Commented May 14, 2019 at 10:15
  • $\begingroup$ @DavidRobinson: Good question and comments, thanks. I used Hay & Davies sky model for every calculation, with irradiance data from PVGIS. It isn't a clear sky model, so diffuse irradiance is taken into account. I'll include the sources in the answer. $\endgroup$
    – Eric Duminil
    Commented May 14, 2019 at 10:20
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Cleaning issues aside, the best orientation of a solar panel is the one that maximizes solar exposure.

It can be fixed or change periodically if a motor mechanism is provided. If it is static, then it can be horizontal assuming the latitude of the location is close to the equator.

Since India is near the equator, there is no reason to tilt southward except the one you mention (cleaning).

For other locations: same rules, maximize sunlight unless there are issues regarding cleaning. So if you go to the northern hemisphere, the farther north you go, the more the Sun will spend time southward in your sky, so you want to incline your panels accordingly. In the southern hemisphere, this is just the opposite. If you own a car, the panels can be horizontal on the roof or if you can move them and camp, the rules above apply.

Also the farther you are from the equator, the more the planet tilt will have an effect on seasonal variation, meaning even less Sun during the winter, or more during summer than during the rest of the year, i.e. a position farther or closer to the vertical, respectively. If the panels are used occasionally or can be moved, you want to aim at the sun anyway.

The basic principle is that you make sure your panels face the Sun as much as possible during the period of time of your interest. Since the Sun spends as much time in the western as in the eastern part of the sky, whatever the latitude, unless for example there is a building producing shadows that would require you to compensate, it should not tilt either westward nor eastward.

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    $\begingroup$ "Since India is near the equator..." India is a large country. Northern India isn't much nearer the equator than the Mediterranean $\endgroup$
    – David Richerby
    Commented May 12, 2019 at 12:08
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    $\begingroup$ Northern India goes up to ~35°N. Furthermore, depending on the context, it sometimes makes sense to optimize the panels for winter output, on the supposition that there'll be plenty of sunlight in the summer to make up for the in-efficient placement. $\endgroup$
    – ShapeOfMatter
    Commented May 12, 2019 at 16:08
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    $\begingroup$ It's possible that if you care more about consistent output than total output, tilting them according to the angle at winter solstice, rather than average angle, is more effective. $\endgroup$
    – R.. GitHub STOP HELPING ICE
    Commented May 12, 2019 at 16:32
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    $\begingroup$ @David: « Northern India isn't much nearer the equator than the Mediterranean » That is incorrect and rather biased, as you take the northern India and the southern medierranean countries. India is close to the equator, while the South of France clearly is not. For those having doubts, just run Google maps and see what I mean. Now regarding maximizing for winter output, I dont see how this conflicts with saying to maximuze exposition for the period of interest anyway. I could argue the same if you dont use panels during winter at all. $\endgroup$
    – Winston
    Commented May 12, 2019 at 17:42
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    $\begingroup$ @Exocytosis What's wrong with comparing northern India to the southern Mediterranean? Especially when I explicitly said "northern India". Northern India is a part of India; the southern Med is a part of the Med. But, heck, Srinagar is at the same latitude as Cyprus or Malta, which is pretty central as far as the Med goes. You, on the other hand, made the blanket statement that "India is near the equator". I'm very sorry, but much of India is actually rather far from the equator. $\endgroup$
    – David Richerby
    Commented May 12, 2019 at 18:07
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  1. Because in northern hemisphere sun is at the south (it moves from east to west - but always on the south side of the sky), by tilting the solar panel towards the sun (to the south) you increase energy generation. Solar panel at 90° angle towards the sun gives you near 0 energy, at 0° (directly looking at the sun) you are getting maximum rated power. Of course without solar tracking which turns solar panel towards the sun - you will not be at 100% efficiency all the time. But this tilt increase your results in average.

  2. It is directly relevant: At the equator your optimal angle is around 0° as sun rises to zenith, at 45° latitude optimal angle is around 37°. So maximum and average sun elevation angle directly depends on latitude.

You can see more on angle variation here: https://www.solarpaneltilt.com/

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    $\begingroup$ @BarsMonster: "always on the south side of the sky" This is false, it depends on your latitude. $\endgroup$
    – Winston
    Commented May 12, 2019 at 9:14
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    $\begingroup$ @Exocytosis That is correct, it's only in Northern hemisphere. $\endgroup$
    – BarsMonster
    Commented May 12, 2019 at 10:10
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    $\begingroup$ ...and between the tropical circles part of the year. The sun's position varies throughout the year because of the tilt of Earth's axis. $\endgroup$
    – Moyli
    Commented May 12, 2019 at 13:42
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    $\begingroup$ @BarsMonster The Sun can be due north of you in the Northern hemisphere, if you’re north of the Arctic Circle. $\endgroup$
    – Mike Scott
    Commented May 12, 2019 at 16:45
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    $\begingroup$ In every single location on Earth, from the South Pole to the North Pole, there's at least one time of year during which the sun in the northern part of the sky. Could you please remove or modify the first sentence? $\endgroup$
    – Eric Duminil
    Commented May 13, 2019 at 9:22
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Its because the sun is never quite in the same part of the sky during the year. Different tilts will generate more energy at different times of year - eg a report for the island of Sark gave 2 tilt options:

enter image description here

So 60 degree tilt gives more in winter, but less in summer, with a small loss of total efficiency. No doubt India is different for solar generation, but that could be why panels are tilted like they are.

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