Factors affecting roof area required by rooftop solar PV plants

The extent of roof area required by a solar PV plant is dependent on two factors

• Shade-free roof area
• Panel efficiency

Shade-free roof area

Unused rooftop area will have to be assessed for incidence of shadows through the year to determine the extent of shade-free area available for installing a rooftop solar PV plant.
We emphasise shade-free roof area because shadows affect the PV plants’ performance in two ways

• Output – When a shadow falls on a PV panel it reduces the output from the plant
• Panel damage –When a shadow falls on part of a panel, that portion of the panel turns from a conductor into a resistance and starts heating up. That portion of the panel will eventually burn out and the entire panel will have to be replaced. This will not be covered by warranty

It is therefore critical to ensure that no shadow falls on the PV plant throughout the year. Shadows that fall on the plant can be from

• Neighbouring structures –Buildings, hoardings, mobile phone towers, and even trees can cast a shadow on a rooftop PV plant
• The PV plant itself –One row of panels can cast a shadow on the row behind them; the further we move away from the equator, the longer the shadow that is cast and the greater the amount of room required between rows of panels

Panel efficiency

Panel efficiency influences rooftop space requirement because efficiency is calculated with respect to the area occupied by the panel. We have a more detailed discussion on panel efficiency here, but a simple way to understand the relationship between panel efficiency and rooftop space required is to remember that a rooftop plant that uses panels with a lower efficiency rating will require greater rooftop space than a plant that uses panels with higher efficiency rating.

Shade-free area required at different plant capacities and panel efficiencies

If a 1 kW plant with 15% efficiency panels requires 100 SF of rooftop space, then a 1 kW plant with 12% efficiency panels will require 125 SF of rooftop space. We can extend this to different combinations of rooftop plant capacity and panel efficiency for our understanding.

Note: These numbers are indicative only. Actual roof area required at your installation could vary based on site-specific conditions and vendor’s recommendations.

Based on the above, we can see that a rooftop solar PV system typically requires 100-130 SF (about 12 m2) of shade-free roof area per kW of capacity.

Other considerations

Weight of the rooftop PV plant

Rooftop solar PV plants are fairly heavy (about 30-60 Kgs/m²). They do not pose a problem for concrete roofs but cannot be installed on asbestos roofed sheds. Metal roofed facilities may or may not be able to withstand the weight and wind load and will need to be assessed by an expert.

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Mountings that can withstand wind pressure

Rooftop solar panel mountings would need to withstand wind pressure building up under the panels during storms. This is an important consideration if you are located in a region prone to cyclones. 2009’s Cyclone Aila, with wind speeds up to 120 kph, took away about 60,000 solar power systems attached to homes in the Sunderbans; the recent Cyclone Phailin brought winds of up to 200 kph. The kind of mounting required for your location and type of roof should be discussed with the installer.

 

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