IRR and NPV for Solar Cashflows

IRR and NPV turn a long stream of solar cashflows into decision metrics. They answer different questions.

NPV: value in today's money

Net present value discounts future cashflows to today's value using a chosen discount rate. If NPV is positive, the model says the project creates value above that required return. A higher discount rate makes future solar savings less valuable, so NPV falls as the discount rate rises.

IRR: implied annual return

Internal rate of return is the discount rate that makes NPV close to zero. It is useful because many investors think in annual return terms. However, IRR can be misleading when cashflows change sign more than once or when projects are very different in size.

Why both matter

Project A can have a high IRR but small absolute profit because it is small. Project B can have a lower IRR but much larger NPV because it deploys more capital at an acceptable return. Solar buyers should look at both return rate and total value.

Cashflow timing also matters. A project with more savings in early years can look stronger than a project with the same total lifetime savings spread later. Financing, maintenance, inverter replacement, tariff escalation, and degradation all affect the timing of cashflows.

Inputs that move IRR and NPV

• Initial EPC cost.
• Self-consumption ratio and retail electricity price.
• Export tariff or net metering credit.
• Annual degradation and equipment availability.
• Debt ratio, interest rate, and repayment term.
• Tax and incentive treatment.

Use discount rate intentionally

The discount rate should reflect the user's required return or opportunity cost, not a random default. A household comparing solar to low-risk savings may choose one rate; a business comparing solar to other capital projects may choose another. Changing this rate is a useful way to test whether a positive NPV is robust.

Use IRR to compare return efficiency. Use NPV to estimate value creation under your required return.