What is inside a rooftop plant - and what to check
Two quotations for the same capacity can differ by tens of thousands of rupees, and the difference is almost never margin. It is in the components - and in what one of them has quietly left out. This guide walks through the plant part by part, so you can read a bill of material the way an engineer does.
How the system fits together
Sunlight hits the modules, which produce direct current. The inverter converts that to alternating current at grid voltage and frequency. That AC feeds your own loads first, and anything spare flows through the net meter into the grid.
The components, one by one
Solar modules
The largest single cost in the plant. Current generation modules are N-type TOPCon, in the range of roughly 545 to 620 Wp each. What to check: the make and model named in the bill of material, MNRE / ALMM approval, and the warranty split - typically around 12 years product warranty and 25 to 30 years performance warranty. If a quotation says only "solar panels" with a wattage, you cannot compare it against anything.
Grid-tied inverter
Converts DC to grid-synchronised AC, and it is also the brain of the plant - maximum power point tracking, grid protection, anti-islanding and monitoring all live here. It is the component most likely to need attention over 25 years. Check the make, that it is ISI standard and MNRE approved, the warranty period, and whether monitoring is included or an extra. A slightly oversized inverter relative to array size is normal and sensible; a badly undersized one clips your generation on the best days of the year.
Mounting structure
The part that quietly separates a good installation from a bad one, because it is the cheapest thing to compromise on and the most expensive to fix. Ask for:
- Hot-dip galvanised, not merely painted or powder-coated. Galvanising thickness of 60-80 micron is a reasonable specification for a coastal-influenced state like Gujarat.
- Section sizes stated in the quotation - for example 60 x 40 x 2 mm and 40 x 40 x 2 mm pipe.
- Elevated design if you want to keep using the terrace underneath. This costs more than a low-profile structure, and it is usually worth it.
- Wind loading considered for the site, not a standard structure copied from job to job.
DC and AC cabling
Solar-rated, UV-resistant DC cable and appropriately sized AC cable, from a named brand. Undersized cable is invisible on the roof and costs you generation every day through voltage drop. Check the cross-section quoted, and how many metres of evacuation are included - most quotations cover a fixed distance from plant to mains and charge beyond it.
DCDB and ACDB
Distribution boxes housing the protection devices on the DC and AC sides. They should be IP-65 rated for outdoor use, with branded MCBs, fuses and surge protection inside. A quotation that mentions the boxes but not what is inside them is telling you something.
Earthing and lightning protection
Separate earthing for array, inverter and lightning arrester, using copper-coated spike rods with a proper earthing kit - not a rod pushed into the soil. A rooftop array is the highest conductive object on your building, so an ESE-type lightning arrester with adequate coverage is not optional. This section is one of the first things cheap quotations thin out.
Monitoring
A Wi-Fi or GPRS monitoring interface reporting generation to an app. It is the only way you will know your plant has underperformed before your electricity bill tells you three weeks later. Confirm it is included rather than optional.
Balance of system
Conduit, cable ties, clamps, end caps, fasteners. Small money, and the reason a plant either still looks tidy in year eight or has cables sagging into standing water. Stainless steel fasteners are worth insisting on.
Why two quotations differ
| Where the money hides | What to ask |
|---|---|
| Module make and technology | Which make and model? ALMM approved? What is the performance warranty? |
| Inverter make and warranty | Which make? How many years? Is monitoring included? |
| Structure specification | Hot-dip galvanised at what micron? Elevated or low-profile? Section sizes? |
| Cable sizing and length | What cross-section? How many metres of evacuation are included? |
| Earthing and lightning | How many earth pits? Is a lightning arrester included? |
| Net meter and DISCOM charges | Included, or extra? What happens if the meter has to be relocated? |
| Paperwork | Who files the portal application, net metering and subsidy claim? |
| O&M period | How many years free? What does it actually cover? |
A practical test. Ask each contractor for an itemised bill of material naming every make. The ones who send it within a day are the ones who have engineered your system. The ones who resist are quoting a price and will decide the components later - usually after you have paid the advance.
What good looks like on site
- Modules aligned, clamped correctly, with no torque marks or cracked frames.
- Cables run in conduit and tied, not draped across the slab or hanging from the structure.
- DC and AC boxes mounted, labelled and closed, at a height that can be reached for service.
- Earth pits accessible and marked, not buried under later civil work.
- Inverter mounted in shade with clearance around it for ventilation.
- A handover file: as-built drawing, warranty certificates, test reports and monitoring login.
How we quote. Every quotation we issue carries an itemised bill of material with the makes named, the structure specification stated, the evacuation distance defined, and the exclusions listed in the open. Compare it line by line against anything else on your table - that is exactly what it is written for.