There are several forces that may act on a solar panel: its own weight; the weight of precipitation (i.e. snow) on the top of it (which we’re lucky not to deal with in our part of the world); the forces continually applied by wind and rain, and in extreme cases seismic loads as well (especially in places with continued tectonic activity like Japan).
In order for the panel to withstand all these forces, it is absolutely necessary to install a mounting system. Thus, the mounting system provides the structural strength to accommodate the worst-case combination of these forces while at the same time provides the support to place it in the correct position for maximum power output.
The designer of the photovoltaic (PV) system can select between two main options: Ground Mounting Systems and Roof Mounting Systems.
The selection depends on a wide set of variables that include: local requirements associated with PV systems; space; budget; location, client’s preferences and more.
Each type of mounting system offers advantages and disadvantages that must be evaluated for every homeowner to provide the best technical and economical solution. Let’s discuss the decision-making considerations of each type.
Roof Mounting Systems
Your roof can act as the base support structure of the PV system, which is actually the preferred option for homeowners in cities.
This configuration consists of installing a rail system or a set of flashings to attach the panels to the roof.
There are several types of rooftop systems: railed; rail-less, shared-rail and flat roof.
Homeowner and designer can select the best type according to the needs and the available configuration of the roof, although generally, shared-rail systems tend to be more cost-effective.
There are intrinsic advantages and disadvantages associated with this type of system.
Let’s take a look at them.
Advantages of Roof Mounting Solar Systems
- Selection of Azimuth angle
- Installation procedures
- Lower installation costs
- Difficult access to solar PV panels
- Roof protection
- Lower Shading Losses
Selection of Azimuth Angle
The azimuth angle represents the orientation of the PV panels towards the south. In the latitudes below the equator, the higher solar radiation can be achieved by placing the PV system facing north.
Most rooftops have two sides (at least), one in the front of the house and one on the back of the house. Therefore, the designer is able to choose which side of the roof is best for the efficiency of the solar system, if the front side faces north then the front house is the best choice.
Some large houses even have four sides to choose, making it even better for the designer. But it also indicates less roof space on the preferred side.
In the case where the house has two sides and neither one of them faces north then a ground mounted option might be best.
The installation of ground-mounted systems is not easy at all. It involves the evaluation of the soil, the excavation of the ground, the positioning of the foundations, installing the vertical and horizontal supports and then making all the proper electrical installations.
On the other hand, roof-mounted systems are easier to install because the main structure needed to support the panels (roof) is already there.
The installation of the racking system, rails, flashings, bolts, etc., is much easier and faster as once the rails are installed the rest of the panels just need to be placed in their positions. And since most roofs are not that wide, installing a maximum of three rails (shared-railed option) is more than enough.
Lower Installation Costs
As stated above, roof mounting systems are easier and faster to install than ground-mounted systems, which means less labour and time to install the system and therefore lower costs. Ground mounted systems require the installation of foundations to provide stability to the structure. Such foundations can be either piles, screws or ballasted concrete, all of which increase the costs of ground mounting and make roof systems more attractive.
This advantage may be lost if the roof is not suitable for withstanding a set of solar panels.
The maneuverability in the selection of the side roof also offers the possibility of adjusting the solar system to the preferences of the homeowner.
If the homeowner does not like the idea of having solar panels on the front side of the house, then he can place them at the back. And if the homeowner is proud of his renewable energy source, then he can show it at the front of the house.
Hard Access to Solar PV Panels
Placing the solar panels in the highest and most inaccessible part of the house has its advantages, minimizing the access of any kid or intruder to the solar system.
Although it is not the intended purpose of the solar panels, they can also provide protection to the roof against hail, rain, gusts of wind and any possible damage from external objects.
Lower Shading Losses
One of the main factors to diminish the power output of the solar system is the presence of shade on the panels.
Shade block the solar radiation to the panels and therefore makes it impossible for solar cells to produce electricity in the shaded area. Moreover, if the panel is not designed to include a bypass diode, then the whole string of solar cells will lower the current values to the current value of the shaded module, which at the end effects the power output.
Shading studies are amongst the most important considerations when designing a solar system. Often, placing the PV modules on the rooftop minimizes the possibility of other structures blocking the sunlight at some time of the day because they are on the highest part of the house.
Keep in mind that some structures may not be suitable for ground mounting installation as a new tree or structure on the neighbours’ backyard could impose some unforeseen shading in the panels.
Disadvantages of Roof Mounting Systems
- Difficult maintenance
- Perforation of the roof
- Limited system size
- Roof upgrades
- Local restrictions
Unfortunately, a great disadvantage of placing your solar panel on the roof is the maintenance procedure.
Particularly in areas with sandstorms or heavy, long-lasting winters it is uncomfortable for the homeowner to clean the solar panels. This is why the panels are often not cleaned and losses from the resultant soiling can be very dramatic.
Perforation Of The Roof
To attach the panels to your roof it is necessary to drill the structure of the roof. You may think, the more holes in your roof, the less safe it is and besides, there is the possibility of water filtration during the rain if the sealant is not properly placed.
Is important to check with your solar installer that the roof is able to stand up to the weight of the solar system (including the forces of wind and panels’ weight).
Limited Size Of The System
The available space of many rooftops is usually not that big, therefore the dimension of the system is often not determined by the load that the homeowner wants to backup, but by the available area on the roof.
Sometimes rooftop areas also have chimneys or antennas which not only take away physical space but also generate shade which can exclude more area of the rooftop due to efficiency reasons.
Depending on the location of the solar system is possible that local homeowner associations establish rules against the presence of visible solar panels from the street.
That means solar panels will have to be installed on the back side and if it doesn’t face north then solar power output could be low.
The structural roof study is crucial for the design. If the study reveals that the roof is not able to endure the estimated weight of the solar system during its lifetime, then upgrades, reparations and even removals may be needed. If that is the case, then it is probably better to switch to a ground mount option to avoid higher costs.
Ground Mounted Systems
This configuration consists of installing a metal frame that is anchored to the ground in order to provide the stability and support that the solar panels need.
There are several possibilities to install ground-mounted systems: foundation mounts (which at the same time are divided into several options), ballasting footing mounts and pole mounted.
The selection of one or another ground mounted is out of the range of this article but depends mainly on the soil structure.
Advantages of Ground Mounted Solar Systems
- Higher size capacity
- Tracking system installation
- Maneuverability to select tilt and azimuth angle
- Easy to add PV modules
- Wide air flow
- Home improvement is not necessary
Higher Size Capacity
The availability of space makes it suitable for solar panels to install higher nominal power capacity of the system, meaning that the size of the system can be dimension by the desired load itself and not by the maximum power output size of the area.
Tracking System Installation
Ground mounted systems have the possibility to include solar tracking systems in their designs.
These systems allow the PV panels to follow the position of the sun at all times, making it possible to achieve the optimum power output for longer periods.
Maneuverability To Select Tilt And Azimuth Angle
Ground mounted structures tend to offer several fixed possibilities to set the inclination (tilt) and orientation (azimuth) angles.
The chance of selecting the best angles for your system across the year is definitely an advantage as the optimum angles change across the seasons.
Easy To Add PV Modules
Over the lifetime of the modules, the desired load to feed could increase and so an expansion of the system could be necessary.
If so, ground-mounted systems offer an easier possibility of adding new panels in the systems because the space in the yard tends to be bigger than the space left on the roof.
Wide Air Flow
The operational temperature on the PV cells is directly related to the efficiency of the modules. The reason is that as temperature increases, voltage drops, which leads to less power output.
Actually, manufacturers establish temperature coefficients for voltage, current and power values, which reflect a voltage/current/power diminution per temperature unit above the Standard Test Condition temperature (25°C).
Therefore, it is advisable that the solar panels have a good airflow with a good wind speed on both sides of the panel (front and back) to cool down the solar cells.
Ground mounted systems allow this feature while roof-mounted do not, since the underside does not have a wide air flow.
Soiling is one of the factors that negatively influence the overall performance of the PV system because the available sunlight is not completely absorbed. Besides, accumulation of dust in the panels can lead to moisture and eventually leakage of currents and electrical failures.
One of the most remarkable advantages of ground-mounted systems over rooftop is the easy and comfortable maintenance procedure. As they are located on the ground, cleaning the modules from dirt, soil or dust is much easier, which allows regular maintenance routines to be established and improves the overall performance of the system.
Home Improvement Is Not Necessary
If you focus only on the mounting structure itself, then the ground mounted option does not need to modify anything in your house. So, you can keep your house structure intact.
Disadvantages Ground Mounted Solar Systems
- Installation procedures and costs
- Land use
- Shading obstacles
The installation of a ground-mounted system requires the study of the soil, selection of the foundation type, selection of the size of the vertical and horizontal pipes, excavation of the ground, concrete filling, installation of pipes and mounting of panels.
As you can see it is quite a process and varies with every house because of soil conditions and the configuration of the solar system, i.e., number of rows or strings and number of modules per string.
All of this translates into more time and higher costs.
Ironically, another big downside of ground-mounted systems is the area of installation, the backyard.
This is an area typically designed for barbecues, parties and other purposes, so placing solar panels over it removes space that otherwise would be used for recreation.
This, of course, depends on the size of the backyard and on the homeowner’s perspective.
Solar panels that are placed in the backyard tend to be located close to the neighbour’s area.
This creates an uncertainty and could create problems because in the neighbour’s property a new tree could be planted, or a tree house could be constructed, therefore, configurations that were not established at the moment of the installation could cause an unwelcome shade in the future.
Actually, maybe your current neighbour is not interested in planting a tree, but if he moves, then the next owner could be.
So this presents a situation that is uncertain and as the system’s lifetime is around 25-30 years, then it should be considered.
Solar systems carry expensive equipment, therefore it is advisable to keep them as safe as possible from any vandalism.
Placing the solar panels in your backyard makes them more vulnerable than if they were on the rooftop, so if your neighbourhood is not that safe you should consider the roof possibility
Moreover, locating your panels on the ground also presents another problem related to the safety of the children.
While nowadays most PV installations are secured and verified by utility inspectors, they still carry wires with electrical current and the degradation factor can deteriorate the performance and insulation of wires.
In other words, while the possibility that a child could be harmed by playing with wires is low, it is not impossible.
There are several mounting system options available to better suit the homeowner’s needs.
From available options, two big groups (roof mounted and ground mounted) can be chosen, the decision depends on several factors, but how can they be weighed?
The choice you make must be balanced between technical and economic factors, thus, if you are thinking about installing a rooftop mounted system, then you should take into account:
Rooftop Mounted Solar System Considerations:
- Age and condition of your roof
- Strength. Dirt/sand load, wind load, seismic load limits
- Accessibility to the roof
- Membrane type: tile, metal, roll roofing, thermoplastic, concreted, etc. This is important as some roofs like Spanish shingles, are not suitable for racking systems.
- Overall system costs
- Available space on the roof.
- Maintenance procedure (imagine yourself cleaning the panels)
- Inclination and orientation angle of your roof.
On the other hand, if you are thinking about installing a ground mounted system, then:
Ground Mounted Solar System Considerations:
- Soil type
- Drainage structure
- Vegetation control
- Security concerns
- Desired tilt and azimuth angles.
- Neighbour’s consultancy (speak with your neighbour about the topic)
- Shading obstacles (sometimes your house could act as an obstacle)
- Dust and soiling factors.
- Maintenance procedures
- Installation costs.
- Excavation of the ground
- Loss of recreation area
- Available space in the backyard
- The need to install a tracking system
- Future expansion of the system
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