Following are the steps that need to be followed before the rooftop solar system installation to ensure for a reliable performance of the system.
This is the first and most important step as it forms the basis on which the other decisions can be made. Some of the alternatives before you include
- Feeding into the grid – If the state solar policy permits, power generated from your rooftop can be fed into the grid and payment received based on a Feed-in-Tariff (FIT) or net-metering
- Diesel substitution – The plant will need to integrate with the diesel generator and the grid power supply to act as a backup for diesel generator and grid power. Also the inverter should be capable of switching between sources. This solution can be quite complex if multiple diesel generators are used
- Off-grid solution – Used in areas where grid power is absent, this solution requires an off-grid inverter
- Night-time usage – As solar power is generated during the daytime, energy storage solutions will need to be considered as part of the plant rooftop
The amount of energy needed is determined based on the load that needs to be supported. The load represented by the equipment can be calculated as
This should be divided by 1,000 to be converted into kWh/day. We can illustrate this formula by calculating the load for a sample commercial establishment:
|Appliance||Number||Wattage||Working Hours||Energy (kWh/day)|
This establishment would require 36 kWh of power per day to satisfy the load. At this point the plant designer might wish to identify large/variable loads that need not be supported by solar power or that can be operated through some other power source to reduce the investment in the solar system.
Let us assume that we have limited the load to be supported by the solar PV plant to this:
|Appliance||Number||Wattage||Working Hours||Energy (Kwh/day)|
This load requires 11.52 kWh/day.Adding a 30% safety margin to this, and assuming the generation (usable AC power generated by Solar energy) at your location is 4 kWh/kWp/day, we get
We calculate the panel requirement for this system size assuming we are using 250 Wp panels.
At this point the system designer may wish to verify if there is sufficient roof space available for installing fifteen 250 Wp panels. Typically, a 1 kWp system requires 100-130 sq.ft (or 10 sq.mt) so a 3.75 kWp system would occupy about 375-490 sq. ft. SF of shade-free roof area.
If sufficient roof space is not available, the system designer could revisit the loads that need to be supported to determine which critical loads can be supported based on the amount of energy generation that the available roof area permits.
We use a 30-45% safety margin when calculating the inverter size.
Total wattage of appliances is calculated in this table:
The inverter size is smaller than the required solar panel capacity (3,750 Wp); as this is a grid-tied plant we will size the inverter based on panel capacity to avoid throttling plant output.The solar PV system required to power this load would need 15 x 250 Wp panels and an inverter of at least 3.75 kW capacity.
The extent of roof area required by a solar PV plant (and therefore the amount of energy that can be generated) 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. Where string inverters are used, a bit of shadow on one panel can curtail the output from the entire string of panels
- 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
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
Shade-free area required at different plant capacities and panel efficiencies
If a 1 kW plant with 15% efficiency panels requires 100 sq.ft 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 (Sq.ft.)|
Note: These numbers are indicative only. Actual roof area required at your installation could vary based on site-specific conditions and vendor’s recommendations.
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 often cannot be installed on asbestos roofed sheds (will depend on the underlying support structures). 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.
Based on their capacity, inverters can be very heavy. A 100 kW off-grid/hybrid inverter can weigh more than 1 tonne and occupy only a few square feet, creating a load of several hundred kilograms per square foot. Similarly, batteries can also be very heavy, depending on the extent of autonomy required. Such heavy components may need to be installed on the ground, or carefully placed over roof beams and columns to avoid overloading the roof.
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, type of roof, and rooftop solar system design should be discussed with the installer.
When integrating a solar plant with a diesel generator, two factors have to be considered.
- Reverse current – Solar power from the plant may flow into the DG if the load reduces. This can cause the diesel generator to turn off, which in turn will cause the solar PV plant to shutdown
- DG fuel efficiency – The more load supported by the solar plant, the less load on the DG. If the load on the DG reduces beyond a certain extent, the fuel efficiency of the generator will suffer, resulting in increasing diesel consumption
Once the initial assessment of requirements, available space, and generation potential is done, the next step is to speak to solar solution vendors and obtain quotations. This serves the purpose of ascertaining the options available within your budget, and adjusting your assessment for site-specific conditions.
Choosing a good vendor is critical to getting the most out of your rooftop PV system as carelessness in design or construction/installation can either significantly reduce the power output from your plant or deliver a plant that isn’t suited to your needs. A few things to keep in mind when finalising a vendor are
- Supplier Background & Credibility
- Ask for details of projects that they have already implemented
- Check if they are MNRE authorised, or registered under your state’s energy development agency (or equivalent body)
- Check if the supplied products have been manufactured in a ISO-9001 certified plant
- Verify supplier’s claims about the product/component with datasheets available on the manufacturer’s website (e.g., if the supplier claims that the panels are suitable for coastal areas, check the product datasheet to see if it has cleared the salt mist corrosion test)
The cheapest vendor is not necessarily the best vendor. A vendor who has a well-established after-sales service network may quote a higher price but will provide greater benefits in the long run.
- PV Panels – Industry standard warranty is
- 5-year manufacturer’s warranty
- 0-10 years for 90% of the rated output power
- 10-25 years for 80% of the rated output power
- Other systems – Inverters, mounting structures, cables, junction boxes, etc. typically come with a 1 year manufacturer warranty which can be extended to 5 years
- Manufacturer warranties are preferred to dealer warranties
When evaluating different vendors, ensure that the plant specification, and not just the description, is the same. E.g., When evaluating a hybrid power plant, 1 kW panel + 5 kW inverter may be sold as a 5 kW plant but is actually only a 1 kW plant (the inverter can support the load, but the panels will not generate sufficient power). Similarly, 5 kW panels + 1 kW inverter is also a 1 kW plant (panels will generate a lot of power but inverter can support only a small load). Such plants can be offered at a much lower price than a genuine 5 kW plant, but will not generate anywhere near the same amount of power (or support the same load).