Calculate the Right Solar System Size for Your Home
Going solar involves choosing the right number of panels for your roof. If you install too few, you’ll depend on grid power; if you install too many, you might pay for unused capacity.
In India, where you get 6+ hours of strong sunlight daily, rooftop solar can easily cover most of your household electricity needs if it’s sized correctly.
Let’s walk through how to calculate your monthly energy use, choose the right system size, and estimate how much money you could actually save on your electricity bill.
Step 1: Check Your Electricity Bills to Know Your Real Usage
Start by collecting your last 3-6 months’ electricity bills. You’re looking for the value listed as:
- Units Consumed
- Energy Charges (in kWh)
- Sometimes also shown as “Billed Units”
Monthly Usage Estimates
| Home Type | Monthly Units (kWh) | Common Appliances |
|---|---|---|
| 1 BHK Apartment | 100-150 | Lights, fans, TV, fridge |
| 2 BHK Flat | 200-300 | Add washing machine, 1 AC |
| 3 BHK House | 300-500 | 2 ACs, water heater, kitchen appliances |
| Large Bungalow | 500-1000+ | 3+ ACs, borewell pump, inverter battery, geysers |
If you use AC more frequently in summer, take a monthly average instead of using a single bill.
Step 2: Use This Simple Rule to Estimate System Size
A well-performing solar panel setup will generate 4 to 5 units (kWh) of power per kilowatt per day. That means:
- 1 kW system = ~120-150 units/month
- 3 kW system = ~360-450 units/month
Formula to Use:
Required kW = Monthly Units ÷ 120
So if your house uses:
- 240 units/month – install a 2 kW system
- 360 units/month – go for a 3 kW system
- 500 units/month – consider a 4-5 kW system
It’s okay to round up slightly if your usage fluctuates or you expect new loads like an EV or another AC.
Step 3: Check How Much Roof Space You Actually Have
Here’s how much shade-free space you’ll need:
| System Size | Roof Area Needed |
|---|---|
| 1 kW | 80-100 sq. ft. |
| 2 kW | 160-200 sq. ft. |
| 3 kW | 250-300 sq. ft. |
| 5 kW | 400-500 sq. ft. |
Make sure this area is:
- Free of water tanks, satellite dishes, or trees
- Not shaded from 10 am to 4 pm
- On a flat, reinforced RCC terrace (preferred)
If you have a tiled, sloped, or asbestos roof, ask your installer for a custom mount solution.
Step 4: Use a Reliable Solar Calculator to Confirm Your Sizing
Once you’ve estimated your monthly usage and space, plug your numbers into a solar calculator.
Trusted Tools You Can Use:
- https://api.solarrooftop.gov.in/rooftop_calculator (MNRE official calculator)
- Tata Power Solar and Adani Solar sites also offer helpful tools
You’ll need:
- Your location
- Your monthly electricity usage (kWh)
- Your roof area (approximate)
The calculator will tell you how many panels, expected monthly generation, and savings range.
Step 5: Estimate Your Monthly and Long-Term Savings
Electricity in India is billed at:
- ₹6.5 to ₹7.5 per unit (Example)
- A home using 300 units/month will pay ₹2,000-₹2,400 on average
What You Can Save with Solar
| System Size | Monthly Units Generated | Monthly Savings (₹) |
|---|---|---|
| 2 kW | 240 | ₹1,500-₹1,800 |
| 3 kW | 360 | ₹2,200-₹2,700 |
| 5 kW | 600 | ₹3,500-₹4,500 |
This is assuming your system is net-metered and most usage happens during the day.
Solar Savings – Cut Your Bill
- Sunlight: India gets consistent solar exposure throughout the year, even in monsoon
- Net Metering: Some States allows net metering for systems up to 10 kW installer should assist
- Subsidy: PM Surya Ghar Yojana offers up to ₹78,000 off for residential systems (via pmsuryaghar.gov.in)
- Appliance Timing: Run appliances like geysers and washing machines during the day to maximize solar use
- Consider Growth: If you’re planning to install an EV charger, add ACs, or build another floor size your system with that in mind
A rooftop solar system in India is not just a way to save on bills; it’s an investment in energy independence. Use your monthly usage data and the 120 units-per-kW rule to check your calculations with a reliable tool. This ensures you select a system that gives you returns year after year.
