Getting the size right is the single most important decision in going solar. Too small and you keep importing expensive grid power; too large and you waste money on capacity the capped subsidy won't cover. This guide shows you exactly how to size a rooftop solar system for your Chennai home — with a free calculator, the simple math behind it, and the three real-world limits that decide your final number.
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Enter your details below for an instant estimate of the right system size, cost after subsidy, and expected savings. It uses Chennai-specific generation assumptions.
Solar System Size Calculator
Estimates use Chennai averages (≈120 units/kW/month) and indicative 2026 pricing. Your exact size depends on a rooftop survey. This is a guide, not a quote.
How Solar Sizing Actually Works
There's no single "right" size — it's the point where three limits meet: how much power you use, how much roof you have, and your budget. Get any one wrong and the system underperforms or overspends. Let's take them in order.
Step 1: Start With Your Electricity Usage
Your usage is the anchor. Pull your average monthly consumption in units (kWh) from your TANGEDCO bill — ideally an average across a few months, since summer AC use inflates a single bill. In Chennai, each 1kW of solar generates roughly 120 units per month (about 4–5 units per day). So the core formula is:
Example: 540 units ÷ 120 = 4.5 kW
This sizes the system to cover close to 100% of your consumption. Some homeowners deliberately size slightly under to keep costs down, or slightly over if they expect usage to grow. Here's a quick reference:
| Monthly Units | Recommended Size | Typical Home |
|---|---|---|
| Up to 250 | 2 kW | 1–2 BHK, light usage |
| 250–400 | 3 kW | 2–3 BHK, one AC |
| 400–550 | 4 kW | 3 BHK, two ACs |
| 550–700 | 5 kW | 4 BHK / villa |
| 700+ | 5 kW+ / custom | Large villa, heavy AC |
Step 2: Check Your Roof Space
Usage tells you what you want; the roof tells you what's possible. You need roughly 80–100 sq ft of shadow-free area per kW. On compact Chennai rooftops, this often becomes the real ceiling — a home that "needs" 5kW by usage may only fit 3kW once you account for water tanks, staircases, and shade from neighbouring buildings.
| System Size | Roof Space Needed |
|---|---|
| 2 kW | ~160–200 sq ft |
| 3 kW | ~200–250 sq ft |
| 4 kW | ~300–350 sq ft |
| 5 kW | ~350–400 sq ft |
Higher-efficiency panels (mono PERC, TOPCon) fit more capacity into less space — useful when the roof is tight.
Step 3: Factor In the Subsidy Cap and Budget
Budget is the third limit, and the subsidy shapes it in a way many people miss. The PM Surya Ghar subsidy is capped at ₹78,000 for any system 3kW or above. That makes 3kW the best value-per-rupee size — it captures the full subsidy at the lowest cost. Going bigger is worth it when your usage genuinely needs it, but "oversizing just in case" means paying for the extra capacity almost entirely yourself. We break this down fully in our 3kW vs 5kW comparison.
Step 4: Adjust for the Future
Sizing is a snapshot, but you'll live with the system for 25 years. If you know you'll add an electric vehicle, a new air conditioner, or extend the home, build that in now — retrofitting extra capacity later costs more than including it upfront. An EV alone can add 150–300 units a month depending on how much you drive. If nothing's changing, size to today's usage and keep the cost down.
Common Sizing Mistakes to Avoid
| Mistake | Why It Hurts |
|---|---|
| Sizing from one summer bill | Overshoots your real annual average |
| Oversizing "to be safe" | Subsidy is capped — you pay full price for the excess |
| Ignoring shade | Real generation falls short of the spec |
| Undersizing to save upfront | You keep importing costly top-slab grid units |
| Forgetting future loads | Expensive to expand a live system later |
Get an Exact Size from a Free Rooftop Survey
The calculator gets you close. A quick site survey confirms your true usable area, shading and the right system for your Chennai home — free and with no obligation.
Book My Free SurveyFrequently Asked Questions
How do I calculate what size solar system I need in Chennai?
Divide your average monthly units by about 120 (each 1kW generates ~120 units/month in Chennai). For 450 units that's ~3.75, so a 3–4kW system. Then confirm your roof has ~80 sq ft of clear space per kW.
How many units does 1kW of solar generate in Chennai?
Roughly 4–5 units per day, or about 120–150 units per month, thanks to strong year-round sun. Output depends on orientation, tilt and shading.
How much roof space do I need per kW?
About 80–100 sq ft of shadow-free roof per kW. A 3kW system needs ~200–250 sq ft; a 5kW needs ~350–400 sq ft.
Should I size for current or future usage?
Size for current usage plus known additions like an EV, a new AC or an extension. Oversizing wastes money (the subsidy is capped); undersizing leaves you importing costly grid power.