How HOMER Calculates IRR

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The Internal Rate of Return (IRR) is the discounting rate that makes the Net Present Value (NPV) equal to zero.

$$\sum_{N=0}^{n} \left( \text{nominal cash flow} \times \frac{1}{(1 + i)^N} \right) = 0$$

(This is how Excel calculates IRR if you use its function on the Net Operating Income column.)