Start with your bill, not with panels
Your last twelve months of electricity bills tell you almost everything a designer needs. Average monthly units, not rupees, determine system size. In Trichy, one kilowatt generates roughly 4.2 to 4.5 units per day across the year, so a home consuming 400 units a month needs around 3 kW to cover most of it.
Compare like for like
Two quotes at the same price can differ enormously. Check the module make and whether it is ALMM-listed, the inverter brand and warranty term, the structure height and galvanisation grade, cable make and sizing, and whether earthing, lightning arrestors and net-metering charges are included or added later.
Structure height is not a detail
A low structure is cheaper but wastes your terrace. An elevated structure at 6 to 10 feet costs a little more and keeps the space underneath usable for drying, parking or a seating area. It also improves air flow under the modules, which raises generation slightly in Tamil Nadu's heat.
Read the warranty carefully
A 25-year warranty on the module means performance, not free replacement of everything. Ask separately about product warranty, inverter warranty, structure warranty and the installer's own workmanship warranty — and confirm who you call when something fails in year seven.