Why Rooftop Solar Systems are Generating Less Than Ground Mounted Solar System and How can we Improve this by a Better Solar System Design

Gyan Prakash Tiwari | 27th February 2017

When solar system designers estimate the energy generation potential for your rooftop solar system they generally apply the same system design rules as they would do for large-scale ground-mounted systems. As a result, when the system starts generating, the actual generation falls short of the predicted numbers, leading to dissatisfied and unhappy consumers. In this blog, we not only try to explain why perhaps your rooftop solar systems are generating less but also tell you how a good system design can ensure that the solar system on your roof performs equal to or in some cases, even better than the large ground mount systems. First, lets try to understand the reasons or factors that could be leading to lower generation. Some of them may be beyond our control but many can be controlled through good engineering, project execution and O&M practices.

Factors that may be beyond our control

Roof orientation – In ground mount system we can optimize the orientation of the modules (Tilt angle and azimuth angle of module) in order to capture maximum solar irradiance whereas in rooftop (primarily in sloped roofs) installations, the angle and orientation of the roof may not be ideal in order to capture maximum solar irradiance.
Availability of space – Due to space constraint in rooftop installations, solar system designers compromise with the ideal pitch (distance between two rows of solar panels) which leads to higher energy loss because of shading.
Temperature and distance between Solar Panels and Roof – We know that solar photovoltaic modules have a negative temperature coefficient. In simple terms, this means that output of solar system decreases with increase in ambient temperature around the solar panel. If the solar panel is too close to the roof, there is less air-flow to cool down the solar cells, and therefore the ambient temperature around the panels tends to increase leading to a loss in generation. Ground mounted systems have a better ground clearance between the solar panel and ground, whereas rooftop systems sometimes are installed very close to the hot surface of roof and therefore the generation from rooftop systems is lowered further..
Maintenance/ System oversight/ etc.- Large ground mounted solar power plants normally have better access to preventive and corrective maintenance activities as they can afford to have trained teams on the site. However, due to smaller system sizes in case of rooftops, it is not possible to keep full time on-site engineers. However, this situation can be improved by having a robust remote plant monitoring and analysis in place as well as having a firm O&M contract with the solar system installer.

Factors that can be controlled

Design & Engineering- This is one of the most important factors which is widely ignored by a majority of solar system installers. A sound solar system design is a must to get the best possible performance from a rooftop solar system and most of the solar system installers lack in this area. A poor design leads to a substantial loss of generation and therefore impacts your savings and financial returns negatively. How do we ensure optimum solar system design and engineering?

Site assessment – Site assessment is very critical for any solar installation whether it is a rooftop or ground mounted system. Ground mounted systems generally have a free horizon and no nearby shading objects whereas rooftops are prone to near and far shading objects like chimneys, overhead water tanks, air handling units, trees, nearby buildings, etc. In order to avoid the impact of these shading objects, a detailed shading analysis is required to find out best possible location on roof to install your solar system. If this analysis is not done properly, you may lose a large amount of generation. Do note that even a shade on 5% of a single module may lead to a 75-80% loss of generation in the solar string, which that that module is a part of. MYSUN takes a lot of pride in their detailed site assessment efforts. You can read more on it, in the Customer Services section here.

Solar Plant Layout – The layouting and placement of solar equipment on your roof is extremely important for an optimum performance as the solar radiation is not uniform across the rooftop area. A proper layout can also help reduce the cost of your solar system. It is advisable to connect electrically those modules in same string which are prone to early shading compared to the other modules which are not. One should segregate the modules basis their current rating and try to keep modules with a narrow current range in one string. You can also put lower current rating strings in lesser solar access areas whereas higher current rated strings can be placed in higher solar access areas. This way, one can optimize the system output within the given rooftop circumstances. Ambient Solar Panel/Module temperature is a big concern for rooftop systems, so you should try to elevate your structure from roof surface to make a provision for proper air circulation in order to reduce module temperature and therefore get better generation. Moreover, try to place inverters at a central location of module array area to minimize cable length which may further reduce your ohmic loss. In ground mounted system, we generally do not see such problems, as sites are quite open and have almost uniform solar irradiance.
Selection of Equipment- If we select right equipment considering the local weather and site conditions, generation loss can be reduced. Solar modules with a better temperature coefficient can reduce your temperature loss, use of string inverter with multi MPPT can reduce mismatch and shading losses, use of appropriate size of cable can reduce your ohmic losses.

Execution- If you take care of a few small things during installation of your solar system, you can improve the performance.

Segregate the modules on the basis of their current rating and try to put modules with similar current rating in one string. This would be helpful to reduce your module mismatch losses.
Keep the provision of air circulation in order to control the module cell temperature in order to reduce losses due to temperature.
Connect the early shadowing strings of array on separate MPPT and the strings shadowing later on a separate MPPT of inverter.
Try to avoid any joints in the cable used in entire solar plant.
Use adequate size and material of cable lugs to make electrical connections. Proper crimping tools should be used for crimping.
All electrical connection should be air-tight to improve your plant availability.
Use proper grounding of equipment and lightning arrestors.

Operation & Maintenance (O&M) - Though solar plants do not require much O&M , but a good planning and preventive schedule backed with a detailed plant performance data can help improve the generation significantly. In the case of large ground mounted solar power plants, the systems are monitored and maintained by on-site skilled manpower, whereas rooftop systems lag in this regard, as most of them either do not have monitoring systems in place or do not get monitored at all. In absence of monitoring, it would be very tough to track plant performance closely and it may lead to a substantial loss of generation. We strongly recommend that each and every rooftop solar system, howsoever small in size, should use a data logger or a remote monitoring device. If we take care of simple things, generation can be improved significantly:

Periodic cleaning of Modules – By cleaning of modules as per recommended cleaning cycle, we can easily increase our generation by 2-7% or even more depending on location of installation.
Temporary shading object- Sometimes, we put some temporary objects like cables/ wires crossing across the roof, cloth stands, etc. which may cast shadow on modules and lead to generation loss.
Visual inspection of panels for bird droppings, fallen leaves, dirt accumulation, cracks or any damage
Inspections for Inverter function by observing the LED indication and Meters etc.

With the aforementioned pointers, we can say that there are certain constraints in roof top system design which may reduce the generation as compared to ground mounted systems, but if we do proper engineering and system designing and keep certain factors in our mind during installation and O&M, this gap can not only be eliminated but we can actually get a much higher performance from our rooftop solar system.

Run our Advanced Solar Calculator where you can change your roof area, shading, orientation and roof to check the generation that you can realistically expect from your rooftop solar system. Make sure you visit the MYSUN blog often and follow us on our Social media profiles to ensure that you keep up with more such interesting articles on rooftop solar.

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Gyan Prakash Tiwari is a Co-Founder at MYSUN. He is a veteran with more than 20 years of experience in solar system engineering. He has worked on more than 200 roof-top and ground-mounted solar systems around the globe and can be found at https://www.linkedin.com/in/gyan-prakash-tiwari-97760a1b

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