Choosing the right solar system size is the most important step before you buy panels. Size affects how much you save. Size affects your bills. Size affects whether you need batteries. Size also affects how long your system pays back. This guide explains the whole process in plain English. It uses simple math. It gives India-specific tips. It also points out common mistakes and how to avoid them.
Why system size matters
A correctly sized system gives you the most savings. A too-small system leaves you buying grid power every month. A too-large system wastes money and may cause export problems with your DISCOM. The right size matches your daily energy need (kWh) and your roof area. It also fits local rules like net metering and subsidy limits. Knowing the right size helps you get better ROI and fewer surprises.
Basic terms you should know (short and clear)
Kilowatt (kW): Unit of power. Panel wattage is shown in watts (W). 1,000 W = 1 kW.
Kilowatt-hour (kWh): Unit of energy. Your electricity bill shows kWh per month.
Peak Sun Hours (PSH): Hours of equivalent full sun per day. It tells how strong the sun is in your city. India usually has 4–6 PSH.
Performance factor: Fraction of energy you actually get after real-world losses. Typical values are 0.70–0.82 for Indian homes.
DC:AC ratio (ILR): Ratio of panel DC capacity to inverter AC rating. Rooftops commonly use 1.1–1.3. Solar 91Tata Power
Step 1 — Find your real energy need (kWh/day)
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The first step is reading your electricity bills. Take 12 months of bills if possible. Add them and divide by 12 to get monthly use. Then divide by 30 to get daily average kWh. This is your true energy need. If your bills vary seasonally, note the peak months. Use those to size systems that must cover peak needs, or plan a partial offset if you want lower upfront cost.
Example: Your bills show 360 kWh per month. Daily average = 360 ÷ 30 = 12 kWh/day.
Do not estimate by family size alone. Real bills give real needs.
Also Read Maximize Your Savings: How to Calculate Solar ROI for Your Home in India
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Step 2 — Know your location’s sunlight (Peak Sun Hours)
Peak Sun Hours (PSH) simplify sunlight into an easy number. PSH is how many hours per day your site gets the equivalent of 1,000 W/m². India’s PSH varies by city and season. Most Indian cities average 4–6 PSH. Use reliable local sources or solar maps to get a number for your city. When in doubt, ask your installer for a PSH value they used in the yield model. Solar 91Tata Power
Quick rule: Use 4.5–5 PSH for conservative planning in many Indian cities. Use 5–5.5 PSH in sunnier regions like Gujarat or parts of Rajasthan.
Step 3 — Pick a performance factor for real losses
Raw panel power at the lab is not the power you get on your roof. Losses come from temperature, dust, wiring, inverter inefficiency, and shading. This combined effect is called the performance factor. For dusty, hot rooftops use 0.70–0.75. For cleaner, cooler roofs use 0.78–0.82. Your installer should justify the chosen factor in the energy model.
Step 4 — Use the sizing formula
Now you can estimate the DC system size in kW with one formula. It is simple.
DC system size (kW) = Daily energy need (kWh/day) ÷ (PSH × Performance factor)
Example with our earlier numbers:
Daily need = 12 kWh, PSH = 5, performance = 0.75.
DC size = 12 ÷ (5 × 0.75) = 3.2 kW. Round up to 3.3 or 3.5 kW to allow small headroom.
This calculates the total panel watts you need under STC divided by 1000 to get kW.
Step 5 — Convert kW to number of panels
Panels come in different watt ratings. Common home panels in India are 375 W, 400 W, and up to 450 W or more. To find the number:
Panels needed = DC system size (kW) × 1000 ÷ Panel wattage (W)
If you need 3.3 kW and choose 430 W panels: 3,300 ÷ 430 ≈ 7.7 → use 8 panels.
Higher-watt panels mean fewer modules on the roof. This is useful on small roofs. But check panel dimensions and weight before deciding.
Step 6 — Check your roof area and orientation
Measure usable roof area. Consider shadow from tanks, chimneys, trees, and neighbouring buildings. South-facing roofs give the best annual yield. East–west layouts are common in many Indian homes. They produce a flatter daily curve and often match home use better.
A rule of thumb: 1 kW requires about 6–10 m² (65–110 sq ft) depending on panel efficiency and spacing. If your roof is small, choose higher-efficiency modules. If you have ample roof, you can use lower-watt modules for lower cost per panel. Always draw a layout before ordering. Tata Power
Step 7 — Check net metering rules and subsidy caps
Net metering policy is set by your state DISCOM. Each state has its own caps, settlement rules, and export tariffs. Some states allow annual carryforward of credits; others have monthly settlement or specific export limits. These rules change ROI and affect how much export you should plan for. Always check your DISCOM’s current policy before finalising size. PM Surya Ghar and other central schemes also define subsidy caps and eligible sizes. If you plan to use these subsidies, your system must meet program rules and use approved products. Solar TigerPMSuryaghar
Step 8 — Decide whether you need batteries (and how that affects size)
If you expect frequent outages, you may want a battery. Batteries let you use solar at night and give backup. But batteries add cost and change the sizing logic. If you add a battery, size the PV to both meet daily use and to recharge battery reliably in your PSH. For backup-only needs, you can choose a smaller battery and a smaller PV array. For long night use or high EV charging, you will need a larger array and battery.
Hybrid inverters simplify battery integration. If you might add storage later, choose an inverter that is battery-ready. This avoids replacement later and reduces retrofit costs. Solar Ace Energy
Step 9 — Choose inverter size and DC:AC ratio
Inverter sizing is a strategic decision. Many installations oversize the panels slightly relative to the inverter. This DC:AC ratio of 1.1–1.3 increases yearly harvest by capturing more early and late sun. It may clip a little at peak noon on cool bright days. Work with your installer to choose a ratio that matches your goals. For most homes 1.15–1.25 is a safe range. Confirm the inverter has enough MPPT inputs if your roof has multiple orientations. Bluebird SolarFootprint Hero
Step 10 — Account for future changes
Think ahead. Will you buy an EV? Add a new AC? Move the home? If you plan growth, size a little larger now or choose a system that can be expanded later. An expandable hybrid inverter and modular battery system give flexibility. Also keep in mind roof changes. If you may re-roof, plan removal and reinstallation costs.
Common mistakes and how to avoid them
Mistake: Sizing by kW only. Fix: Size by kWh/day from bills.
Mistake: Ignoring PSH and performance losses. Fix: Use local PSH and a conservative performance factor.
Mistake: Forgetting DISCOM rules or subsidy limits. Fix: Check net metering and PM Surya Ghar details first.
Mistake: Choosing too-large panels that don’t fit the roof. Fix: Mock-layout on roof before purchase.
Mistake: No plan for inverter replacement or battery retrofit. Fix: Buy quality inverter and battery-ready models.
Practical worked example — a 4-step sizing walkthrough
You can use this as a template.
- Gather bills: Monthly units = 480 kWh → daily = 16 kWh.
- Local PSH = 5 hours (city average). Use performance = 0.75.
- DC size = 16 ÷ (5 × 0.75) = 4.27 kW. Round up to 4.5 kW.
- Panels: choose 420 W panels → 4,500 ÷ 420 ≈ 11 panels. Check roof area for 11 panels and confirm shading clearance.
Then confirm net metering and subsidy eligibility. Check inverter size for ILR ~1.2 (so 4.5 kW DC / 3.7–4.0 kW inverter). Decide on battery only if you need backup. This workflow gives a realistic first design. Solar 91
How to get a reliable proposal from installers
Ask for a written energy model. It should list: assumptions (PSH, performance), monthly and annual kWh, panel make/model and wattage, inverter model, DC:AC ratio, expected annual degradation, O&M costs, and warranty details. Get at least two quotes and compare them on expected annual kWh and warranty, not only price. Ask for references and a site visit to one of their installations nearby. Use the MNRE ALMM list when checking module brands if you want subsidy eligibility and quality assurance. Ministry of New and Renewable Energy
Ongoing checks after installation
Enable monitoring. Many inverters have apps that show real-time power and daily kWh. Check daily curves to spot dips from shading or soiling. Clean panels when output drops steadily over dry weeks. Keep commissioning reports, serial numbers, and warranty cards safe. Plan an annual inspection and tighten mounting bolts, check earthing, and inspect connectors. These steps protect yield and warranty.
Cost and subsidy notes for India
The PM Surya Ghar scheme and some state top-ups can reduce your upfront cost. These schemes have eligibility rules and caps. For many household sizes, subsidies apply up to 2–3 kW with fixed amounts per kW. Using a subsidy can shorten payback significantly. But subsidy rules change, so confirm current details on the official portal or your DISCOM before signing a contract.
Quick checklist you can use right now
- Collect last 12 months of electricity bills.
- Calculate daily average kWh.
- Ask an installer for local PSH and a performance factor.
- Compute DC size with the formula.
- Convert to panel count using your chosen panel wattage.
- Check roof space and shading.
- Confirm net metering rules and subsidy eligibility.
- Choose inverter size (watch ILR) and battery readiness.
- Get at least two detailed quotes and compare expected annual kWh.
- Enable monitoring and log production after commissioning.
FAQs (short answers for Indian homeowners)
Q: Should I size my system to cover 100% of my electricity?
A: You can, but many homeowners size to cover daytime use and buy less than 100% to avoid export complications. Check net metering rules first.
Q: How many kW do I need for a 2BHK or 3BHK?
A: It depends on usage. A 2BHK with 200–300 units/month often needs 2–3 kW. A 3BHK with 400–500 units/month may need 3–5 kW. Always use actual bills.
Q: Can I use an east–west roof?
A: Yes. East–west arrays give flatter daily generation. They may increase panels needed for the same energy. Use multiple MPPTs or split strings.
Q: What if I have limited roof space?
A: Choose high-efficiency panels (higher W per m²). Consider a partial system sized to critical loads.
Q: Do I need ALMM panels to get subsidies?
A: Yes. Many government programs require ALMM-listed modules for subsidy eligibility. Check the MNRE ALMM list before buying.
Final words
Sizing a rooftop solar system is not guesswork. It is a simple mix of your energy needs, local sunlight, and real losses. Use bills, not rules of thumb. Check net metering and subsidy details early. Choose inverter and panels with an eye on future needs like batteries or EV charging. Good sizing gives better savings, fewer headaches, and a happier rooftop for 20–25 years.
If you want, I can build a ready-to-use spreadsheet that takes your monthly bills and gives a suggested system size, panel count, inverter options, and rough cost estimate for your city in India. Which city are you in?
