Plan your rooftop solar
Solar System Size Calculator
Estimate solar capacity, annual generation, roof space, and nearby common system sizes from your electricity use or monthly bill.
Smart result
Your solar size estimate will appear here
Start with your monthly bill or exact electricity units. We will explain the answer step by step.
Quick answer
This calculator estimates the rooftop solar size, in kW, that could produce about as much electricity in a year as your home uses. Enter your monthly units or your bill and rate. It also shows expected generation and the roof area needed, so you know what to ask an installer before getting quotes.
Real-world example
A home uses 300 units a month, using the general India planning assumption of about 1,275 units per kW per year after losses.
- Monthly use
- 300 units
- Yearly use
- 3,600 units
- Generation per kW
- 1,275 units/year
Your home may need about 2.8 kW of solar.
That needs roughly 226–282 sq ft of shade-free roof. The nearest common size, 3 kW, could make about 3,825 units a year.
General planning assumption, not a forecast for your city. Shading, tilt and direction change real output.
How it is calculated
Your monthly use is turned into yearly use, then divided by the electricity one kW of panels may produce in a year after system losses. kW is the system's capacity; kWh (units) is the energy it makes over time. Roof area is the system size multiplied by a planning range per kW.
Solar kW = (monthly units × 12) ÷ (1,500 × (1 − 15% losses)); roof = kW × 80–100 sq ft
Results are planning estimates, not engineering designs, quotations or eligibility decisions.
Assumptions
- Each kW is assumed to produce 1,500 units a year before 15% losses: a general India estimate, not city-level data.
- Roof space is planned at 80–100 sq ft per kW of shade-free area.
- The size aims to match yearly use; it is not an engineering design, quotation or subsidy decision.
What can change the result?
- Your real usage
- Use 12 months of bills if you can; summer-only bills overstate the size you need.
- Location and season
- Sunshine differs across India and through the year, changing units per kW.
- Shade and roof direction
- Trees, water tanks and nearby buildings can cut output sharply.
- Export and net-metering rules
- If surplus units earn little, a slightly smaller system may pay back faster.
- Future loads
- An EV, AC or induction cooking can raise use; plan for it now if likely.
What should I do next?
Frequently asked questions
How many kW of solar do I need?+
Divide annual electricity use by expected annual generation per kW after losses. The result is a planning estimate that needs a site assessment.
Can solar eliminate my entire electricity bill?+
Not necessarily. Fixed charges, export rules, seasonal variation, shading, and the timing of consumption can leave a remaining bill.
How much roof area is required?+
This calculator shows a configurable area range per kW. Panel dimensions, walkways, tilt, obstacles, and setbacks change the final requirement.
Does location affect output?+
Yes. Solar resource, weather, tilt, orientation, temperature, and shading affect generation. Current results use a general India estimate, not city-level data.