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What factors affect roof area required?
The extent of rooftop area required for solar PV power plants has been the subject of interest worldwide.
In the case of large, ground-mounted power plants, these are usually located in lands far from cities and hence are available economically. Hence, the area required per MW is if interest, but not of great concern.
For rooftop solar power plants however, area is a significant constraints in most cases, for both residential and commercial rooftop solar power 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. In many cities worldwide where both residential and commercial buildings have a number of adjoining buildings and other structures, shading analysis will be an important aspect to consider before estimating the actual area available for rooftop solar.
- 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. 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.
Want to figure out how efficiency of panels affect area required by solar plants in an interesting way? Click Here
What we have seen so far is that, in some countries such as the United States, there are a number of rooftop solar installations that are using high efficiency solar panels (for instance from companies such as SunPower), in order to derive the most from the limited rooftop area. However, as of 2015, the high efficiency panels (21-22% efficiency) cost a lot more per kWh delivered than the medium efficiency panels (16-18%). As a result, very high efficiency panels are used for rooftop only in select countries; in countries such as India, rooftops still use the medium efficiency panels and not the high efficiency ones.
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.
| Plant capacity | 1 kW | 2 kW | 5 kW | 10 kW |
| Panel efficiency | Rooftop space required (SF) | |||
| 12.0% | 125 | 250 | 625 | 1,250 |
| 12.5% | 120 | 240 | 600 | 1,200 |
| 13.0% | 115 | 231 | 577 | 1,154 |
| 13.5% | 111 | 222 | 556 | 1,111 |
| 14.0% | 107 | 214 | 536 | 1,071 |
| 14.5% | 103 | 207 | 517 | 1,034 |
| 15.0% | 100 | 200 | 500 | 1,000 |
| 15.5% | 97 | 194 | 484 | 968 |
| 16.0% | 94 | 188 | 469 | 938 |
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 SF (about 10 m2) of shade-free roof area per kW of capacity. |
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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.
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. Recent hurricanes in the United States and cyclones in many other parts worldwide have sensitised installers and rooftop solar owners to be more careful about the mounting structure materials and designs. The kind of mounting required for your location and type of roof should be discussed with the installer.
Takeaways
- Rooftop Solar PV plants require about 100 sq.ft (10 sqm) of shade-free roof area per kW of plant capacity
- Shadows falling on the panels not only reduce power output but also damage the panel
- Rooftop plants weigh 30-60 Kgs/m2 (66-130 lb/m2) which is too heavy for asbestos roofed sheds. Installation on metal roofed sheds should be decided on a case-to-case basis
- The mounting structure should be designed to handle cyclones where wind speeds can reach 200 kph (125 mph)
