Choosing between a 3kW and 5kW rooftop solar system comes down to two numbers you already have: your average monthly electricity consumption and your available shadow-free roof space. Get the sizing wrong in either direction and you either pay for capacity you don't use, or end up with a system too small to meaningfully cut your TANGEDCO bill. Here's how the two sizes actually compare for a typical Chennai home.
Quick Comparison
| Factor | 3kW System | 5kW System |
|---|---|---|
| Ideal for | 2–3 BHK home, 300–400 units/month | Larger home or high AC usage, 500+ units/month |
| Roof space needed | ~300 sq. ft. shadow-free | ~500 sq. ft. shadow-free |
| Approx. units generated/month | ~360 units | ~600 units |
| Cost after subsidy* | ~₹1.45 lakh | ~₹2.62 lakh |
| PM Surya Ghar subsidy | ₹78,000 (max tier) | ₹78,000 (capped at 3kW+) |
| Typical payback period | 5–6 years | 5–7 years |
*Indicative pricing based on 2026 market rates for on-grid systems; actual cost depends on panel brand, inverter type, and site conditions. Get an exact quote from a site assessment.
Which One Fits Your Home?
Match your system size to your actual electricity usage, not just your roof size.
3kW System
- Best if your monthly bill runs 300–400 units and you live in a standard 2–3 BHK independent house or flat.
- Generates roughly 360 units a month — close to fully offsetting a typical Chennai household's usage.
- Needs about 300 sq. ft. of clear, shadow-free terrace space, achievable on most independent house roofs.
- Qualifies for the full ₹78,000 PM Surya Ghar subsidy, the same top-tier amount as larger systems.
5kW System
- Best if your bill runs 500+ units a month — common in larger homes, homes with multiple ACs, or homes planning to add an EV charger.
- Generates roughly 600 units a month, giving more headroom against rising summer AC loads.
- Needs about 500 sq. ft. of clear roof space — worth confirming before committing, since Chennai terraces often have water tanks, staircases, or AC units eating into usable area.
- Same subsidy cap applies at ₹78,000, so the extra 2kW of capacity is paid for entirely out of pocket, not subsidized further.
How to Size Your System Correctly
- Check your last 12 months of electricity bills — take your total annual units and divide by 12 for a true monthly average, since usage spikes in summer months.
- Divide by ~120 — that's roughly how many units 1kW of solar generates per month in Chennai, giving you your baseline system size.
- Measure your actual shadow-free roof area — exclude space taken up by water tanks, staircases, parapet walls, and AC outdoor units, not just total terrace size.
- Account for future usage — if you're planning an EV, a second AC, or an addition to the home, sizing slightly larger now avoids a second installation later.
- Get a professional shadow and load analysis — rooftop obstructions and orientation can change your effective output more than the system size on paper.
Common Questions
What Chennai homeowners usually ask before deciding on system size.
Can I start with 3kW and add more panels later?
Yes, in most cases you can expand a system later, but it's more cost-effective to size correctly from the start — a single site visit, inverter, and net-metering approval is cheaper than two separate installations.
Will a 5kW system eliminate my electricity bill completely?
It depends on your consumption pattern and how much of your usage happens during daylight hours versus night. A correctly sized system paired with TANGEDCO net-metering can bring your bill close to zero, but exact results depend on a site-specific load analysis.
Does a bigger system mean a bigger subsidy?
No. Under PM Surya Ghar, the subsidy caps at ₹78,000 for systems of 3kW and above, so a 3kW and 5kW system receive the same maximum subsidy — the extra capacity of a 5kW system is self-funded.
What if my roof can't fit a 5kW system?
If shadow-free space is limited, higher-wattage panels can generate more capacity per square foot than standard panels, which is worth discussing during your site assessment before ruling out a larger system.