Solar ROI Calculator
Calculate payback period and returns from a rooftop solar installation.
Calculator
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Annual generation: 150563 kWh
Annual saving: ₹13,55,063
Payback period: 4.4 years
Formula
Annual generation (kWh) = System capacity (kWp) × Performance ratio × Annual peak sun hours × 365. Annual savings = Generation × Tariff. Payback = System cost / Annual savings.
Example calculation
100 kWp rooftop system, performance ratio 0.75, 5.5 peak sun hours/day (India average): Annual generation = 100 × 0.75 × 5.5 × 365 = 1,50,562 kWh. At ₹9/kWh: savings = ₹13.6 lakh/year. System cost ₹60 lakh: payback ≈ 4.4 years.
Engineering notes
Performance ratio (PR) accounts for temperature losses, soiling, wiring losses, inverter efficiency, and shading. Typical PR in India: 0.70–0.80. Peak sun hours vary: Rajasthan 6.0, Maharashtra 5.5, Tamil Nadu 5.2, West Bengal 4.8. Degradation: solar panels lose ~0.5% efficiency per year. Factor in O&M cost of ₹0.30–0.50/kWh for a realistic analysis.
When to use this calculator
- Industrial rooftop solar — evaluate feasibility and financial returns for a factory rooftop PV system
- RESCO model evaluation — compare self-ownership vs developer-owned solar with PPA pricing
- Bank financing — prepare financial model for solar project loan application
- RPO compliance — calculate how much solar generation is needed to meet Renewable Purchase Obligation targets
- Carbon reduction — quantify CO₂ savings from solar to support ESG and sustainability reporting
Frequently asked questions
- What is a realistic payback period for industrial rooftop solar in India?
- For HT industrial consumers paying ₹8–10/kWh: typical payback is 3.5–5 years for rooftop solar systems installed at ₹40–50/Wp (2024 prices). After payback, the system generates essentially free electricity for 20+ more years. IRR is typically 18–28% for well-designed systems, which is attractive compared to other capital investments.
- What is the difference between kWp and kW for solar systems?
- kWp (kilowatt peak) is the rated output of a solar panel under Standard Test Conditions (STC): 1000 W/m² irradiance, 25°C cell temperature, AM1.5 spectrum. In actual installation conditions, output is typically 75–85% of peak due to heat, soiling, and angle of incidence. kW is the instantaneous actual output. A 100 kWp system typically operates at 60–80 kW during midday and much less in the morning, evening, and cloudy conditions.
- Should I choose a net metering or captive consumption model?
- Captive consumption (direct use of solar generation): highest savings because you displace expensive grid power at full tariff rate. Net metering (export to grid): surplus energy exported at a feed-in tariff, typically ₹2–4/kWh — much less than what you import. Design solar to match your daytime load profile to maximise self-consumption. Export only surplus that cannot be stored or used.
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