The Savings-to-Investment Ratio (SIR) is the ratio of the present value of energy cost savings from a project to the present value of the investment costs. A SIR greater than 1 indicates the savings exceed the investment over the project’s lifetime.
SIR = Present Value of Savings / Present Value of Investment
Here is a link for further reference: https://betterbuildingssolutioncenter.energy.gov/content/savings-over-investment-ratio-calculator-sir-tool?utm_source=chatgpt.com
You can also review ANSI/ASHRAE Standard 100-2018