📐 Solar Panel Size Calculator

The Sizing Math (Worked for a ₹3,000 Bill)

Here is the exact computation the calculator runs, using a ₹3,000 monthly bill at ₹8/unit in a state with 4.5 peak sun hours:

1. Monthly units: ₹3,000 ÷ ₹8/kWh = 375 units/month
2. Daily units: 375 ÷ 30 = 12.5 units/day
3. System size: 12.5 ÷ (4.5 peak sun hours × 0.85 system efficiency) = 3.27 kW → rounded to 3.5 kW
4. Panel count: 3,500W ÷ 540W per panel = 6.48 → 7 panels
5. Roof area: 3.5 kW × 100 sq ft/kW = ~350 sq ft
6. Cost before subsidy: 3.5 × ₹50,000 = ₹1,75,000
7. PM Surya Ghar subsidy: ₹78,000 (capped at 3 kW, so the extra 0.5 kW gets no subsidy)
8. You pay: ₹1,75,000 − ₹78,000 = ₹97,000

Panel wattage based on 540W mono PERC modules common in India's ALMM-approved list (MNRE, 2026). Roof area benchmark: ~100 sq ft per kW including mounting clearance. System efficiency factor 0.85 accounts for inverter losses, wiring, temperature derating, and soiling.

Bill-to-Size Reference Table

Match your monthly bill to the system size you need. All figures use ₹8/unit and 4.5 peak sun hours (India average). Panel count uses 540W modules.

The ₹3,000 row is highlighted because a 3-3.5 kW system is where most Indian households land, and it's the last size that captures meaningful subsidy. Above 3 kW, every extra kilowatt costs ₹50,000 full price with zero additional subsidy.

Peak Sun Hours by Region — Why Your State Matters

The same electricity bill needs a different system size depending on where you live. India averages 4.5 peak sun hours/day, but the spread is wide:

A ₹3,000 bill in Rajasthan (5.0 PSH) needs a 3 kW system. The same bill in Kerala (3.8 PSH) needs a 3.5-4 kW system — because each panel generates less per day. Adjust the peak sun hours field in the calculator to match your state.

The Common Mistake: Sizing to Bill, Not to Roof

People size their system to their bill, then discover their roof is too small (or too shaded) to fit the panels. A 5 kW system needs ~500 sq ft of shadow-free, south-facing roof — many urban townhouses don't have that. Check your roof first: measure the usable south-facing area, divide by 100, and that's your maximum kW. If that's less than what your bill needs, install the maximum your roof allows and offset the rest with net metering.

Edge Case: When Your Bill Is Already Low

If your monthly bill is under ₹800 (roughly 100 units), solar may not make economic sense yet. The fixed costs of installation, net-metering filing, and minimum system size (usually 1 kW) mean your payback stretches to 4+ years on a small bill. Consider waiting until your consumption grows, or pool your roof with neighbours under a group housing society application.

Decision Framework: Which Size to Pick

• Bill ₹500-1,000: 1 kW. Smallest practical system. Payback ~2 years after subsidy. Good for small homes or rented roofs.
• Bill ₹1,000-2,000: 1.5-2 kW. Captures 60% subsidy rate. Needs 150-200 sq ft roof.
• Bill ₹2,000-3,500: 2.5-3 kW. The sweet spot — captures full ₹78,000 subsidy. Needs 250-300 sq ft.
• Bill ₹3,500-5,000: 3.5-5 kW. Subsidy caps at ₹78,000, so the extra kW is full price. Only size up if your roof and budget allow.
• Bill ₹5,000+: 5+ kW. High consumption homes. Accept long payback on the unsubsidized portion. Check if your DISCOM allows >10 kW residential net metering.

The 2-Minute Self-Check

1. Pull your last 3 electricity bills. Take the highest month (usually summer with AC use) — not the average. Size for your peak month, not your cheapest.
2. Note the units consumed (listed on the bill as "units" or "kWh"), not just the rupee amount.
3. Divide units by 30 to get daily consumption, then divide by your state's peak sun hours × 0.85 to get kW.
4. Measure your south-facing roof in feet (length × width). Divide by 100 to get your max kW.
5. If roof-limited, size to the roof. If bill-limited, size to the bill. Then run it through the calculator above.

Your Next Steps

1. Run the calculator above with your actual bill, rate, and state's peak sun hours.
2. Measure your roof's usable south-facing area to confirm it fits the recommended panels.
3. Check your subsidy at our PM Surya Ghar Subsidy Calculator — remember, the subsidy caps at 3 kW.
4. Get 2-3 installer quotes. Each should specify panel wattage, count, inverter brand, and total cost. Compare per-kW cost (should be near ₹50,000/kW).
5. Register at pmsuryaghar.gov.in to claim your subsidy.

See your full 25-year savings at the Solar ROI Calculator, or check export credits at the Net Metering Calculator.

Editor Note

Reviewed June 2026. Panel wattage (540W) based on common ALMM-listed modules in India. Roof area benchmark of 100 sqft/kW is a planning estimate - actual area depends on panel dimensions and mounting system. Peak sun hours are regional averages. If you spot an outdated figure, please contact us.