How to Size a Rooftop Solar System From Your Electricity Bill (India Guide)

Sizing a rooftop solar system is mostly a math + practicality exercise: your electricity bill tells you how many units you use, and your city’s sunlight decides how many units a solar plant can generate.

This guide shows you how to calculate the right kW (kWp) size using your bill— plus roof area, inverter, and (optional) battery sizing tips for Indian homes.

 

What you need from your electricity bill

Look for these fields (names vary by DISCOM):

• Units / Energy consumed (kWh) for the billing period
• Billing days (28–32 days typically)
• (Optional) Sanctioned load / connected load (useful for net-metering limits in some states)

Important: use at least 6–12 months of bills

Your usage changes with:

• Summer AC
• Winter geyser/heater
• Guests, WFH, appliance upgrades

If you only use one month, you’ll likely oversize or undersize.

 

Step 1: Find your average monthly consumption (units)

Take total units for the last 12 months and divide by 12.

Example
If your last 12 bills add up to 3600 units, then:

Average monthly consumption = 3600 ÷ 12 = 300 units/month

If you don’t have 12 months, use at least 6.

 

Step 2: Decide how much of the bill you want solar to cover

Common targets:

• 70–80% offset → best if you have daytime usage + want lower export dependence
• 90–100% offset → works best with net metering and stable daytime loads

Practical note: Many homes overshoot generation in some months and undershoot in others. Aiming for 80–90% is usually a safer “feels right” number.

 

Step 3: Use the right solar generation value for your city (kWh per kW per day)

In India, a rooftop system typically generates around:

• 3.5–4.0 kWh/day per kW (lower sun / winter-heavy / pollution / shading)
• 4.0–4.5 kWh/day per kW (most major cities)
• 4.5–5.2 kWh/day per kW (high-sun zones)

Quick reference (use as a starting point)

• 4.0 → Delhi NCR, Kolkata, Lucknow (conservative)
• 4.2–4.5 → Mumbai, Pune, Hyderabad, Bengaluru, Chennai (typical)
• 4.6–5.0 → Jaipur, Ahmedabad, Nagpur, many parts of Rajasthan/Gujarat (high sun)

If you want a “safe” estimate for sizing, use 4.0.

 

Step 4: Calculate solar plant size (kW)

Main formula (simple and reliable)

• Solar size (kW) = Monthly units you want to cover ÷ (30 × daily units per kW)

Example 1: 300 units/month, target 90%, using 4.0

• Units to cover = 300 × 0.9 = 270
• Size = 270 ÷ (30 × 4.0)
• Size = 270 ÷ 120 = 2.25 kW

Round to a standard size: 2 kW or 2.5 kW

If you have AC and future growth, 2.5 kW is usually better.

Example 2: 500 units/month, target 85%, using 4.5

• Units to cover = 500 × 0.85 = 425
• Size = 425 ÷ (30 × 4.5)
• Size = 425 ÷ 135 = 3.15 kW

Choose: 3 kW or 3.3 kW (common module counts make 3.3 kW very practical)

 

Quick cheat sheet (very rough, for 4.0 units/day per kW)

Approx monthly units a 1 kW system can generate:

1 kW ≈ 120 units/month (using 4.0)

So:

• 2 kW ≈ 240 units/month
• 3 kW ≈ 360 units/month
• 5 kW ≈ 600 units/month
• 10 kW ≈ 1200 units/month

If your city is sunnier (4.5), increase these by ~10–15%.

 

Step 5: Check if you have enough roof area

Typical roof area needed in India (with modern mono panels):

• 1 kW → ~80–100 sq ft
• 3 kW → ~250–300 sq ft
• 5 kW → ~400–500 sq ft

Add extra space if:

• you need walkway space
• the roof has shading (overhead tanks, parapet walls, nearby buildings)
• you want tilt frames for winter performance

Rule: If your roof has regular shading, reduce expected generation (or don’t count shaded area).

 

Step 6: Don’t ignore shading and orientation (it changes sizing)

Before finalizing kW, check:

• Shade between 9:30 AM – 3:30 PM (most critical hours)
• South-facing is ideal, but east-west also works well on many terraces
• Avoid placing panels behind parapet walls or water tanks where shadows fall

Shading tip: If part of the roof is shaded, it’s better to size based on the usable unshaded area rather than “total roof area”.

 

Step 7: Choose inverter size (simple guidance)

For Indian residential on-grid/hybrid systems:

• Inverter size is often equal to solar kW or slightly lower (DC oversizing is common)

Examples:

• 3.3 kW solar → 3 kW inverter is common
• 5.5 kW solar → 5 kW inverter is common

Your installer may propose higher panel capacity than inverter capacity (DC/AC ratio). That’s normal when done within safe limits.

 

Optional: If you want battery backup, size it from your “backup load”, not your bill

Your bill shows total energy usage. Battery sizing depends on:

• what you want to run during outages (fans, lights, fridge, Wi-Fi, etc.)
• for how many hours

Battery sizing shortcut

Battery (kWh) = Backup load (kW) × Backup hours ÷ usable fraction

Usable fraction depends on battery type and settings:

• Lithium: often 0.8–0.9 usable
• Lead-acid: often 0.5–0.6 usable

Example: You want 0.6 kW backup for 4 hours with lithium (0.85 usable)

Battery = 0.6 × 4 ÷ 0.85 = 2.82 kWh → choose ~3 kWh

 

Common mistakes people make when sizing from the bill

1. Using only one month’s units (summer/winter mismatch)
2. Ignoring billing days (28 vs 32 days changes the math)
3. Assuming 1 kW = fixed units everywhere (sunlight differs by city + season)
4. Not accounting for shading (even small shadows hurt generation)
5. Oversizing without checking net metering / sanctioned load limits (rules vary)

 

A practical “best-fit” method most Indian homes can use

If you want a dependable approach without overthinking:

1. Average your last 12 months units
2. Target 80–90% offset
3. Use 4.0 as a conservative generation factor
4. Round to the nearest common size: 2 kW / 3 kW / 3.3 kW / 5 kW

This usually lands very close to the “right” size for most rooftops.

If you’re planning rooftop solar seriously and want a broader, decision-level overview, you should also read this detailed guide: Indian Homeowner’s Solar Guide (2025)

Want to run the numbers for your own home?
I’ve put together a simple step-by-step guide + an Excel Solar PV Design Calculator to help you estimate system size (kW), 1st-year generation (kWh/year), savings, and payback.
Read it here: Solar PV System Design Calculator (Excel)

This guide explains the complete rooftop solar journey for Indian homes — including system types (on-grid, off-grid, hybrid), net metering basics, inverter selection, battery considerations, costs, and common mistakes to avoid. It’s a useful companion to this sizing guide, especially if you’re still deciding what type of solar system suits your home.