Rooftop solar, bought well.

Rooftop solar is a twenty-five year decision made in about three weeks, usually on the strength of one or two quotations. This guide is the checklist we'd want a member of our own family to work through before signing — how to size the system, what the components actually do, how the subsidy works, and the specific things that separate an honest quotation from an expensive one.

Start with the bill, not the roof

Almost every bad solar purchase begins with the same sentence: "We have a big terrace, so let's fill it."

Roof area sets your maximum system size. Your electricity consumption sets the right one. In Tamil Nadu the distinction has real financial teeth, because surplus you export but never consume is eventually settled at the avoided cost rate — well below the retail tariff you were hoping to save. We cover that mechanism in detail in the TANGEDCO net metering guide.

So begin here:

1. Collect twelve months of TANGEDCO bills. Total the units consumed across the year. Seasonal swing matters — a summer of air-conditioning distorts any single month.
2. Find your sanctioned load on the bill, in kW. Your system cannot exceed it without a separate load-enhancement application.
3. Divide. In Madurai, each kW of well-installed rooftop solar generates roughly 4–4.5 units per day averaged across the year, monsoon included. A 3 kW system therefore produces somewhere around 360–400 units a month.
4. Aim to offset 85–95% of annual consumption, not 150%.

Our savings calculator does this arithmetic for you, including the split between self-consumed and exported units.

Is your roof suitable?

Four things decide this, in order of importance.

Shadow-free area

Budget roughly 70–100 sq ft of unshaded roof per kW, depending on panel efficiency. A 3 kW system needs about 250–300 sq ft of genuinely clear area — and "clear" excludes water tanks, stairwell headroom, parapet shadows in December, and the neighbour's under-construction second floor. Add margin for walkways and service access; our roof area calculator includes that allowance.

Shading

Shade is not linear. A single shaded panel in a conventional string can drag down the output of the entire string. If your roof has unavoidable partial shading — a tank, a tree, an adjacent building — that's an argument for microinverters or optimisers, not for accepting a lower yield.

Structure and waterproofing

An RCC roof generally takes a non-penetrating ballasted structure or properly sealed foundations. Sheet roofs need gasketed, leak-proof clamps. Either way the mounting must be engineered for wind load — Madurai gets strong seasonal winds, and a structure designed by eye rather than by calculation is a genuine liability.

Orientation

South-facing is ideal in India; east-west splits are workable and sometimes preferable if your consumption peaks in the morning and evening. A good designer models this rather than assuming.

The components that matter

Panels

Monocrystalline panels are more efficient and produce more per square foot — which is exactly what you need when roof area is the binding constraint. Most modern installations use them. Polycrystalline is marginally cheaper per watt and less efficient; on a constrained rooftop it usually costs you more in forgone generation than it saves in capital.

Expect a performance warranty of the order of 25 years, typically guaranteeing around 80–85% of rated output at year 25. Panels degrade slowly and predictably; that's the one part of the system that genuinely lasts.

Inverter

The inverter converts DC from the panels to AC for your home, and it is the component most likely to need replacing once during the system's life — commonly after 8–12 years. It is not the place to economise. Our guide to choosing an inverter works through the decision in full.

Battery — only if you need it

A standard grid-tied system shuts down during a power cut, for the safety of line workers. If riding through outages matters to you, you need a hybrid inverter and storage. If it doesn't, a battery adds substantial cost for benefit you won't use.

Where storage is warranted, lithium (LFP) is now the sensible default: longer cycle life, much higher usable depth of discharge, compact and low-maintenance. Lead-acid is cheaper to buy and considerably more expensive per usable unit over its life — you need roughly double the rated capacity for the same backup. Size it honestly with our battery backup estimator.

Mounting, cabling and protection

The unglamorous half of the system, and where cheap quotations quietly cut. Galvanised or aluminium structures rated for wind load. Properly sized DC cable with UV-rated insulation. Surge protection, earthing, and an accessible AC isolator. None of this is optional; all of it is invisible on a brochure.

Subsidy and net metering

Residential rooftop systems may qualify for the central PM Surya Ghar subsidy, administered nationally with TANGEDCO acting as the state nodal agency. Applications for both subsidised and non-subsidised installations go through TANGEDCO's Unified Online Solar Rooftop portal (USRP), which also handles the net meter.

Three things are worth stating plainly:

• Subsidy is paid to you, after commissioning. Typically you pay the vendor in full and the subsidy is later transferred directly to your bank account. Budget for the full amount upfront.
• Eligibility is specific. The electricity account holder and property owner generally need to be the same person, and the metering arrangement must be DISCOM-approved. Tenants are usually not eligible.
• Commercial and industrial consumers do not receive this subsidy, though they benefit from accelerated depreciation, and typically see faster payback anyway because of higher tariffs and daytime load — see our commercial solar ROI guide.

How to read a quotation

A quotation you can act on names things. A quotation designed to be hard to compare uses categories. Insist on the former.

Twelve questions to ask an installer

1. What is my sanctioned load, and does this system stay within it?
2. How many units per year will this generate — and is that a modelled annual average or a peak-day figure?
3. Which panel and inverter, by make and model number?
4. Who files the TANGEDCO application and manages technical approval?
5. Is the mounting structure designed for wind load, and by whom?
6. How is the roof waterproofed at every penetration?
7. What exactly does the workmanship warranty cover, and for how long?
8. What happens if generation falls short of your estimate?
9. Who services the system, and what does the O&M contract include?
10. Is the inverter's monitoring accessible to me, or only to you?
11. What is excluded from this price?
12. Can I see a system you commissioned two or more years ago?

That last one is the most revealing. A new installer can show you a shiny month-old install. Ask for an old one.

Red flags — walk away

• A quote larger than your sanctioned load, with no mention of load enhancement.
• "We'll just export the extra" used to justify oversizing. Surplus beyond your consumption is settled at avoided cost, not retail.
• Generation figures with no stated assumptions. Ask for units per year, and how monsoon was modelled.
• Pressure to sign before you verify the subsidy on the official portal.
• Unnamed components. "Tier-one panels" is not a specification.
• A price far below every other quote. The saving is coming from somewhere — usually the structure, the cabling, or the paperwork you'll end up doing yourself.
• No written O&M terms. Twenty-five years is a long time to rely on goodwill.

Frequently asked questions

What size system do I need?

Size from twelve months of bills, not roof area. In Madurai each kW generates roughly 4–4.5 units per day averaged annually. Target 85–95% of annual consumption, and never exceed your sanctioned load.

How much roof area does it need?

Roughly 70–100 sq ft of shadow-free roof per kW. A 3 kW system typically needs about 250–300 sq ft, plus margin for access and walkways.

Mono or poly panels?

Monocrystalline, in almost every rooftop case. It generates more per square foot, which is what matters when area is the constraint. Poly's small price advantage rarely compensates.

Will solar run my house during a power cut?

Not a standard grid-tied system — it shuts off for safety. You need a hybrid inverter with battery storage for outage backup.

How long does installation take?

A residential rooftop system in Madurai is usually commissioned within one to three weeks of approval. The approvals, not the installation, are what set the timeline — see our EPC process guide for the stage-by-stage breakdown.