Solar IRR Calculator

How IRR is calculated from annual cashflows

This calculator builds a yearly cashflow series. Year 0 is the initial investment (negative), plus any upfront grant (positive). Each year then includes revenue and costs, with optional replacement and residual value.

• Production: kWh_y = kWh₁ × (1 − d)^y−1
• Tariff: p_y = p₁ × (1 + g)^y−1
• Revenue: R_y = kWh_y × p_y + Incentives
• Net cashflow: CF_y = R_y − Costs_y − Replacement_y ± Taxes + Residual
• NPV: NPV(r) = Σ CF_y / (1 + r)^y
• IRR: The rate r that solves NPV(r) = 0.

IRR may be “not solvable” when cashflows never change sign, or when multiple sign changes create multiple possible rates.

Project Inputs That Drive IRR

Solar IRR is most sensitive to the upfront investment, the expected annual production, and the value of each kilowatt-hour. In construction planning, keep CAPEX aligned with scope items such as structural steel, electrical balance-of-system, permitting, and commissioning. Capture any upfront grant as a Year 0 offset because it immediately improves returns. When comparing bids, normalize to the same system size and scope.

Energy Yield and Degradation Assumptions

The model starts with Year 1 production and applies an annual degradation factor to reflect module aging, soiling, and availability. Typical degradation ranges from 0.3% to 0.8% per year, but site conditions can push it higher. Use measured irradiance, shading studies, and realistic downtime allowances. Small changes in degradation compound over time and can shift payback by multiple years on long-life assets.

Revenue Structure and Tariff Escalation

Revenue can represent avoided utility purchases, export credits, or contract payments. The tariff escalation rate accounts for expected price movement across the project life, often 0% to 4% annually. When tariff escalation exceeds cost escalation, the margin typically improves later in the term. Incentives can be entered as a fixed annual amount or a per‑kWh rate with optional escalation.

Cost Planning and Replacement Events

Annual costs include O&M plus other items like insurance, monitoring, land lease, and cleaning. Many owners budget O&M near 1% to 2% of CAPEX each year. Escalate costs to reflect labor and materials inflation. A scheduled inverter replacement is modeled as a one‑time outflow in a chosen year, commonly year 10 to 15. Including this event prevents overestimating IRR for systems with mid‑life component swaps.

Interpreting Outputs for Construction Decisions

IRR is the discount rate that makes the project NPV equal zero, while NPV uses your selected discount rate for comparison to alternative investments, 6% to 12%. The cashflow table supports procurement decisions by showing when costs spike and when benefits accumulate. Use the export files to document assumptions and review them with stakeholders and finance teams.

FAQs

What cashflows does the calculator include?

Year 0 includes CAPEX minus any upfront grant. Each operating year includes revenue from energy value and incentives, less O&M, other costs, taxes if enabled, and any scheduled replacement. The final year can include a residual value.

Why can IRR show “Not solvable”?

IRR requires at least one negative and one positive cashflow. If the project never becomes positive, or if cashflows change sign multiple times, a unique IRR may not exist. Review assumptions or use NPV for comparison.

Should I use revenue or savings for the tariff?

Use the value of one kilowatt-hour to your project: avoided utility cost for self-consumption, or contracted export payment for sales. If both apply, use a blended rate or treat one part as an incentive.

How do I set degradation and escalation rates?

Start with vendor warranties and performance studies for degradation, then adjust for soiling and availability. For escalation, use your internal forecast or contract indexation. Conservative rates reduce risk of overstating returns.

How is payback calculated here?

Payback is the first year when cumulative undiscounted cashflow becomes zero or positive. It is a simple indicator, not a profitability metric. For capital ranking, prefer IRR and NPV with your target discount rate.

Can I model battery storage with this tool?

Yes, approximately. Add storage CAPEX into initial investment, include additional O&M and replacement costs, and reflect added savings by increasing annual kWh value or incentives. For time-of-use dispatch, a granular hourly model is better.