Project Inputs
Example Data Table
| Scenario | Net Capex | Year-1 kWh | Value | O&M | Life | Discount | Debt | Indicative NPV |
|---|---|---|---|---|---|---|---|---|
| Small commercial rooftop | $55,000 | 80,000 | $0.16/kWh | $800/yr | 25 | 8% | 60% | Varies by assumptions |
| Higher tariff and escalation | $55,000 | 80,000 | $0.20/kWh | $900/yr | 25 | 8% | 60% | Typically improves |
| Lower production and faster degradation | $55,000 | 70,000 | $0.16/kWh | $800/yr | 25 | 8% | 60% | Typically declines |
Use the calculator above to compute exact results for your project. The table shows typical input patterns, not guaranteed outcomes.
Formula Used
- Energy(y) = Energy₁ × (1 − d)^(y−1) where d is annual degradation.
- Price(y) = Price₁ × (1 + e)^(y−1) where e is price escalation.
- Revenue(y) = Energy(y) × Price(y)
- Opex(y) = Opex₁ × (1 + o)^(y−1) where o is O&M escalation.
- Dep(y) = NetCapex / DepYears for y ≤ DepYears (straight-line).
- Taxable(y) = Revenue(y) − Opex(y) − Dep(y) − Interest(y)
- Tax(y) = max(0, Taxable(y)) × TaxRate
- EquityCF(y) = (Revenue − Opex) − Tax − DebtService
- NPV = Σ CF(t) / (1 + r)^t where r is the discount rate.
- IRR is the rate where NPV equals zero (iterative solve).
How to Use This Calculator
- Enter installed cost, incentives, and tax credit assumptions.
- Provide year‑1 energy, degradation, and electricity value.
- Add O&M cost and escalation, then set project life.
- Set your discount rate and effective tax rate.
- If using debt, enter debt ratio, interest, and term.
- Press Calculate Cashflows to view results above.
- Use the CSV and PDF buttons to export your report.
For high‑stakes decisions, align assumptions with contracts, local regulations, and professional tax advice.
Purpose of discounted cashflow modeling
Discounted cashflow turns a solar project into a timed stream of benefits and costs. It values each future dollar less than today’s dollar using a discount rate that reflects risk, inflation expectations, and the required return. This approach supports bid evaluation, client proposals, and internal capital budgeting. It also helps quantify uncertainty and supports transparent early reviews with contractors and financiers.
Building reliable energy and price assumptions
Start with realistic year‑one production based on site irradiance, shading, and system performance. Apply an annual degradation rate to reflect module aging. Next, assign a value per kilowatt‑hour such as avoided tariff, feed‑in rate, or contract price, then add escalation to represent tariff growth over the project life.
Operating costs, taxes, and depreciation impacts
Annual operations and maintenance include cleaning, inspections, insurance, monitoring, inverter reserves, and administrative costs. Escalate these costs for inflation. Taxable income is calculated after operating costs, depreciation, and interest, so depreciation timing can materially reduce early‑year taxes. Effective tax rates should match the project owner’s structure.
Financing structure and lender sensitivity
Debt reduces the upfront equity requirement but adds scheduled repayments. Loan interest is typically tax‑deductible, while principal is not. Longer terms lower annual debt service but may increase total interest paid. Use the debt ratio, interest rate, and term fields to test bankability and to see how leverage shifts equity IRR.
Interpreting outputs for decisions
Net present value shows value created after meeting the discount rate; positive NPV generally indicates an attractive investment. Internal rate of return is the break‑even discount rate for the equity cashflows. Payback measures how quickly cumulative cashflows turn positive, while discounted payback adds time value. Export the table to document assumptions and audit results. When comparing scenarios, change one driver at a time and keep units consistent. Use conservative inputs for early screening, then refine with measured production, contract terms, and maintenance quotes. Document sources so stakeholders can reproduce the case independently.
FAQs
1) What does the discount rate represent?
It represents the return you require for the project’s risk and timing. Higher rates reduce present value of future cashflows, lowering NPV and extending discounted payback.
2) Why can my IRR show n/a?
IRR needs at least one negative and one positive cashflow. If incentives and early savings never overcome the initial outlay, or cashflows never turn negative after year zero, the rate cannot be solved reliably.
3) How should I estimate year‑one energy?
Use modeled production from your design software, adjusted for shading, soiling, and downtime. If you have meter data from a similar site, validate the model against actual performance.
4) Is this calculator suitable for PPAs?
Yes. Set the electricity value to your contracted rate, include any escalation, and treat revenue as the payment stream. Keep capex, incentives, and financing aligned with the party who owns the asset.
5) How do incentives and tax credits affect results?
Upfront incentives reduce net capex immediately. The tax credit is modeled as a year‑zero benefit to equity. Real treatments vary by jurisdiction, so confirm eligibility, timing, and transfer rules.
6) What should I export to share with stakeholders?
Export CSV for auditing and scenario comparison. Use the PDF for a concise summary of assumptions and key metrics. Include notes on energy estimates, tariffs, and maintenance sources for transparency.