India is located in the northern hemisphere but closer to the equator between latitudes 6˚ and 36˚N (the longitude boundaries are 68˚ and 98˚E). The central-most state, Madhya Pradesh, is confined within the latitudes 21˚ and 27˚N. The distance of a degree of latitude is about 111 km (69 miles).The terrain of India includes Deccan plateau, the plains land beside the rivers, the Himalayan Mountain ranges in the Northern part and the desert area in the West. The southern region has tropic rainy climate and the northern part is temperate.
In order to get the most from solar panels, they must point in the direction that captures most sunshine. In India, as anywhere in the northern hemisphere, solar panels should face southwards. However, in the southern hemisphere, for example, in Australia panels should point towards north. Here North means the true north – not the magnetic north as pointed by the compass needle.
Magnetic compass does not always point to North. Actually, there are only a few locations on Earth where it points exactly to the True (geographic) North. The direction in which the compass needle points is known as Magnetic North, and the angle between Magnetic North and the True North direction is called magnetic declination.
Fortunately for India, the magnetic declination is rather small. For example, at Delhi the declination is only 0.41˚ east and at Mumbai the declination is 0.58˚ west. It means that the compass needle gives the direction of geographic north is given quite accurately.
How to Determine the True South
There is an easy method to determine the true south: At solar noon, by definition, the sun shines from true south and thus the shadow cast by any object at solar noon will be along true south to true north. The exact time of solar noon is different from the clock noon and changes slightly throughout the year. Solar noon is exactly midway between sunrise and sunset, which can be taken from the local newspaper of the same day. Apart from running from true south to true north, shadows cast at solar noon have the additional distinction of being the shortest shadows of the day.
For fixed tilt angle throughout the year, the angle of the latitude is preferred. This is one fixed orientation where the panel almost always intercepts the greatest amount of solar radiation during the year. Reference 4 claims that minor tweaking can yield 3 – 5 percent extra gain and generally recommend somewhat lower angle for fixed tilt.
However, in general the horizontal tilt of the panels may be adjusted 4 times a year: at the latitude angle in spring and autumn, (Latitude – 15˚) in summer, and (Latitude + 15˚) in winter. Alternatively, one can choose the angle depending upon when the power requirement is greatest. If power shortages are high in summer and the requirement goes high due to the need of running fans, then latitude – 15˚ should be the right choice. Another good way is to adjust angles twice a year for summer and winter seasons. The best time to adjust for summer angle is mid March and mid September for the winter angle. Following the 15˚ plus/minus rule, for Mumbai and Delhi you can set the panel angles as follows:
For Mumbai (latitude: 18˚ 55’N) summer angle 3˚ and the winter angle 33˚.
For Delhi (Latitude: 28˚ 38’N) the summer angle will be 13˚ and the winter angle 43˚.
There are various online tilt angle calculators, mostly optimized for US or western conditions where people are more concerned with high energy requirement in the winter, and are based on different assumptions. In India, on the other hand, people are more worried about power shortage in the summer months and rising power demand mainly due to fans, coolers and ACs. Therefore, one should think before applying these models in Indian conditions. Obviously they give differing optimum angles depending upon the assumptions inherent in their models. Some of them are:
If you apply these models for Mumbai and Delhi, you will get varying angles than the above common rule of ±15˚ compared to latitude. One model even gives a negative angle for Mumbai summer angle, implying slight tilt towards north rather than south! The only point of agreement is that in summer the panels are mounted flatter than latitude and in winter steeper than latitude.
How Critical is the Tilt Angle
Model calculations of the 4th reference mentioned above shows that if 2 axis tracking is taken to give 100% energy, fixed angle panels generate about 71% energy, adjusting two season gives 76% and adjusting four season does not produce any additional gain. Thus, for fixed mounting adjusting the angles twice a year is simple and good enough for maximum gain.
As far the accuracy of angle is concerned, a difference of few degrees does not make any significant difference in the energy collected by the panels.