IRR (Internal Rate of Return)

What IRR is

Internal Rate of Return (IRR) is the annualised rate of return on an investment, calculated as the discount rate that makes the Net Present Value (NPV) of all project cash flows equal to zero. IRR captures the time value of money and produces a single percentage figure that can be compared across investments.

For a solar project, the cash flow stream is the initial CAPEX outflow at year zero, followed by annual revenue and tax benefits less O&M cost across the project life. At the end of the project’s useful life, there may be a salvage value or asset removal cost. The IRR is the implicit return delivered by this cash flow pattern.

IRR is one of the most widely used metrics for evaluating solar projects. It allows direct comparison with other investment opportunities (stock market, real estate, bonds) and with other solar projects, regardless of project size.

How IRR is calculated

The IRR formula:

0 = sum_t [CF_t / (1 + IRR)^t]

Where:

• t is each year of the project life
• CF_t is the net cash flow in year t (positive for inflows, negative for outflows)
• IRR is the discount rate to be solved

The equation is solved iteratively. Microsoft Excel’s IRR function does this directly. Financial software (Aspen, PVsyst, SAM) handle more complex cash flow patterns.

For a typical 100 kW commercial CAPEX solar project in India:

Year 0: CAPEX outflow of Rs 50 lakh (net of subsidies).

Years 1 to 25: Annual cash inflows of Rs 8 to 12 lakh (electricity savings net of O&M).

Plus Year 1: One-time tax savings from Accelerated Depreciation of Rs 9.9 lakh.

End of Year 25: Residual asset value of Rs 1 to Rs 3 lakh.

The IRR for this cash flow stream is typically 20% to 25% post-tax for a taxable corporate buyer.

Typical IRR ranges for Indian solar in 2026

These are illustrative; actual IRR depends on site-specific tariffs, CUF, and project parameters.

What drives IRR in solar

Four main levers determine IRR.

CAPEX: The denominator effect. Lower CAPEX per kWp directly raises IRR. Module cost reduction has been the dominant IRR improver over the past decade.

Generation per kWp (CUF): The numerator effect. Higher CUF (through trackers, bifacial, optimal tilt, better PR) raises annual revenue and IRR proportionally.

Tariff or savings per kWh: For utility-scale, the PPA tariff. For CAPEX consumers, the grid tariff being offset. Higher tariffs deliver higher IRR.

Project life: Longer plant life amortises CAPEX over more years. 25-year plants outperform 20-year plants on IRR.

Secondary factors:

Tax benefits: AD, GST input credit, MAT credit, and other tax incentives.

Subsidy: PM Surya Ghar and other subsidies effectively reduce CAPEX.

O&M cost: Lower O&M raises IRR slightly.

Degradation: Lower degradation rate raises late-year revenue and IRR.

Cost of capital (for project IRR): Lower interest rates allow higher leverage and improve equity IRR.

IRR comparison: solar versus other investments

For a profitable Indian C&I corporate, the post-tax IRR landscape:

Solar CAPEX with tax benefits: 20% to 28%.

Fixed deposits: 6% to 8% post-tax.

Equity (NIFTY 50): 10% to 14% historical post-tax.

Real estate (commercial): 6% to 12% post-tax.

Corporate bonds: 7% to 10% post-tax.

Solar CAPEX with tax benefits often delivers the highest IRR among readily available investment options for Indian corporates. This is why CAPEX solar adoption has accelerated in 2024 to 2026.

For OPEX or RESCO solar (which has lower effective IRR from the consumer’s perspective because no tax benefits accrue), the comparison is to the cost of doing nothing (continuing to buy grid power), not to other investments.

Equity IRR versus Project IRR

For leveraged projects, equity IRR and project IRR differ.

Project IRR: Considers the total project cash flow including debt service. Reflects the project’s underlying return.

Equity IRR: Considers only the equity holder’s cash flows (initial equity contribution, dividend distributions, exit value). Reflects the return to the equity investor.

For a typical utility-scale solar project:

Project IRR: 12% to 15%.

Debt at 9% to 11% (75% leverage).

Equity IRR: 14% to 18%.

The leverage effect amplifies equity returns when project IRR exceeds debt cost.

For CAPEX residential and commercial solar (typically unleveraged), project IRR equals equity IRR.

IRR limitations

IRR has well-known limitations.

Reinvestment assumption: IRR assumes that all interim cash flows are reinvested at the IRR rate. For high-IRR projects, this is rarely achievable.

Multiple solutions: For cash flow patterns with multiple sign changes, the IRR equation can have multiple solutions, making interpretation tricky.

Project size: IRR does not distinguish between a Rs 1 lakh project at 30% IRR and a Rs 10 crore project at 25% IRR. Both have the same IRR ranking, but the larger project may create much more value.

Risk: IRR does not capture risk. A 20% IRR with high uncertainty is less attractive than a 15% IRR with high certainty.

To address these limitations, financial analysts use IRR alongside NPV, payback period, and risk-adjusted metrics.

Common mistakes with IRR

Comparing IRRs across projects of different scales without considering NPV.

Using pre-tax IRR for taxable corporate decisions. Post-tax IRR is the relevant metric.

Ignoring leverage effects when comparing project and equity IRRs.

Forgetting that IRR is sensitive to assumed cash flow timing. Early cash flows raise IRR more than late ones.

Treating IRR as the only investment criterion. Combine with payback period and risk assessment.

Not modelling realistic operating costs and degradation in IRR calculations.

Best practices

Build detailed financial models with realistic assumptions for CAPEX, generation, O&M, degradation, tax benefits, and end-of-life.

Calculate both pre-tax and post-tax IRR. Post-tax is the relevant metric for taxable corporates.

For leveraged projects, calculate both project IRR and equity IRR. Equity IRR is what matters to equity investors.

Use IRR alongside NPV, payback period, and LCOE for a complete picture.

Run sensitivity analyses on key drivers (CAPEX, CUF, tariff, discount rate) to understand IRR vulnerability.

For long-term projects, account for inflation, regulatory changes, and technology evolution in long-horizon IRR calculations.

Standards and references

IRR is a standard financial metric calculated per generally accepted principles. Microsoft Excel’s IRR function is the most widely used tool. Detailed financial modelling for utility-scale projects often uses Aspen, NREL System Advisor Model (SAM), or PVsyst.

Related glossary terms

• Levelised Cost of Energy
• Payback Period
• CAPEX Model
• OPEX Model
• Accelerated Depreciation
• Power Purchase Agreement
• Feed-in Tariff
• Capacity Utilisation Factor
• Performance Ratio

Key takeaways

IRR (Internal Rate of Return) is the annualised rate of return on a solar investment, calculated as the discount rate that makes the NPV of project cash flows equal to zero. For Indian commercial CAPEX solar with tax benefits, post-tax IRR typically ranges from 20% to 28% over 25 years. For residential CAPEX with PM Surya Ghar subsidy, 18% to 28%. For utility-scale developer IRR, 12% to 16%. IRR is driven by CAPEX, CUF, tariff, project life, and tax benefits. The metric should be used alongside NPV, payback period, and risk assessment for complete investment evaluation.