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The right rooftop-solar system is not the biggest one your roof can hold. It is the one that matches your electricity usage, usable roof and actual energy needs.
A proper solar recommendation should therefore not begin with “Do you want 3 kW or 5 kW?” It should begin with your electricity consumption, site conditions, usable rooftop area and what you actually want solar to achieve.
The simple rule
Electricity requirement tells you how much solar may be useful. Your roof, structure and electrical infrastructure determine how much can practically be installed. The right system should make sense on both sides.
How much rooftop solar do you actually need?
There is no universal system size that is correct for every home or apartment. Two houses with similar roofs can require different capacities because their electricity use is different. Likewise, two apartment communities with the same number of flats can have very different common-area loads.
A good sizing exercise brings together three things: electricity requirement, usable rooftop area and the outcome the customer wants from solar.
| What to check | Why it matters |
|---|---|
| Historical electricity consumption | Shows the scale of the energy requirement |
| Daytime electricity use | Helps understand how much solar can be used directly |
| Future electricity demand | Allows for realistic additions such as ACs or EV charging |
| Usable shadow-free roof | Sets the practical physical limit |
| Sanctioned load / demand | Can influence grid-connected capacity and approvals |
| Customer objective | Bill reduction, backup and future expansion can require different designs |
Start with electricity usage — not the size of the roof
Roof area tells you what may physically fit. It does not tell you what capacity is economically useful. For a home, review recent electricity bills and consider realistic future changes such as additional air-conditioning, work-from-home usage, new appliances or EV charging.
For a housing society, start with the electricity connection the solar system is intended to offset. Common-area demand may include lifts, water pumps, lighting, security systems, clubhouses, treatment equipment and other shared infrastructure. The number of apartments alone is not enough to determine the solar capacity.
Right-sized beats maximum-sized
The objective is not to cover every available square foot with panels. It is to install the capacity that creates the strongest practical and financial value for the property.
How much rooftop space does solar need?
As a general planning reference, TNPDCL currently indicates approximately 10 square metres of shadow-free rooftop area for 1 kW of solar. This is useful for an early estimate, but it should not be treated as the final design calculation.
Actual usable area depends on module dimensions, roof shape, orientation, row spacing, water tanks, staircases, parapet walls, access pathways, shading and the selected mounting structure. This is why available roof area and usable solar area are not the same thing.
What makes a rooftop suitable for solar?
A proper site survey should check more than whether panels fit. It should determine whether the rooftop can support a system that is safe, maintainable and capable of generating effectively over the long term.
- Shading: Trees, neighbouring buildings, tanks, lift rooms and parapets can reduce useful generation.
- Roof condition: Waterproofing, damaged sheets, cracks or other major repairs should be addressed before installation.
- Structural suitability: Roof type, building condition, mounting height and wind exposure affect the mounting design.
- Layout: Panel placement should balance generation, safe access and maintainability.
- Electrical routing: Inverter location, cable paths, protection equipment, earthing and interconnection points should be planned before installation.
- Maintenance access: The array should not make tanks, drains, equipment or the panels themselves difficult to reach later.
A well-designed layout may deliberately leave some rooftop area unused. Filling every available space with panels is not automatically better engineering.
Home solar and housing-society solar are sized differently
| Factor | Residential Home | Housing Society |
|---|---|---|
| Primary electricity reference | Household consumption | Relevant common-area electricity connection |
| Typical loads | Household appliances, ACs and future needs | Lifts, pumps, lighting and shared infrastructure |
| Decision process | Homeowner / family | Committee and resident approval |
| Rooftop planning | Usually simpler ownership and access | Shared rooftop use and common services must be coordinated |
| Proposal focus | Personal savings and system suitability | Collective savings, technical feasibility and clear committee justification |
For housing societies, a technically sound design is only part of the requirement. The committee also needs a proposal that clearly explains why the recommended capacity was selected, how much common-area demand it may offset and what responsibilities remain with the association.
Should subsidy decide your solar capacity?
No. Subsidy should improve the economics of the right system rather than determine the system size. A homeowner should not automatically select 3 kW only because that reaches the current maximum residential subsidy band if the actual electricity requirement points to a different capacity.
The better sequence is: understand consumption → check rooftop feasibility → determine the appropriate capacity → calculate the applicable subsidy.
For current Central and Tamil Nadu residential subsidy amounts, refer to the separate Tamil Nadu Solar Subsidy Guide.
What approvals are involved in Tamil Nadu?
Grid-connected rooftop solar requires coordination with the electricity distribution utility. The exact workflow depends on consumer category, project size, subsidy participation and the rules applicable at the time of application.
For residential consumers applying under PM Surya Ghar, the current process begins through the National Portal and continues through the Tamil Nadu Unified Solar Rooftop Portal. Non-subsidy rooftop-solar projects follow the applicable Tamil Nadu portal process.
- Up to 3 kW: TNPDCL currently states that technical feasibility is waived for solar applied load up to 3 kW.
- Above 3 kW: the applicable utility feasibility process is required.
- Inspection and commissioning: the installed system proceeds through the required readiness, inspection and grid-connectivity process.
- Above 10 kW: current TNPDCL guidance requires the applicable CEIG approval.
- Metering: the required bi-directional metering and grid connection are completed under the applicable utility framework.
Approval rules can change
Utility procedures, technical requirements, forms and metering rules can be updated. Always verify the latest TNPDCL and Government requirements before finalising a project.
What should happen during a proper solar site survey?
A meaningful survey should produce decisions, not simply rooftop photographs. The installer should understand the electricity requirement, measure usable roof area, evaluate shading and roof condition, plan the mounting approach and identify the electrical connection route.
- Review electricity bills, sanctioned load and major electrical loads.
- Measure the roof and identify genuinely usable areas.
- Check shading from buildings, trees, tanks and other obstructions.
- Assess roof condition and mounting possibilities.
- Identify inverter, protection, earthing and cable locations.
- Plan safe installation and future maintenance access.
- Understand future electricity demand and the customer's objective.
Common mistakes when choosing a system size
- Recommending capacity before reviewing electricity consumption. This often indicates a standard package rather than a site-specific recommendation.
- Sizing only from roof area. Maximum physical capacity is not automatically the right economic capacity.
- Ignoring shading or roof condition. Both can materially affect the usefulness and quality of the installation.
- Ignoring realistic future demand. Near-term additions such as ACs or EV charging may affect the appropriate size.
- Covering the entire roof. Safe access, drainage and future maintenance still matter.
- Presenting approvals as guaranteed. An installer can manage and support the process, but approvals remain subject to the relevant authority.
How should you decide the final solar capacity?
Do not begin with “How many panels can I install?” Begin with “What electricity requirement am I trying to solve?”
Then evaluate electricity consumption, daytime load, future demand, usable roof, shading, structural and electrical feasibility, applicable utility constraints, subsidy where relevant and the overall project economics.
The final system capacity should be the result of that assessment rather than the starting assumption.
How Scaler Solar approaches system sizing
We prefer to understand the requirement before recommending capacity. That means reviewing electricity usage, assessing the site and explaining why the proposed system makes sense for that specific home or housing society.
The objective is not to maximise the number of panels installed. It is to design a solar system that fits the property, produces useful electricity and remains practical to operate and support over the long term.
The principle
Right-sized solar begins with the energy problem — not with a package size.
