Green Net Present Value (NPV) Calculator

Net present value asks whether the discounted stream of benefits exceeds the upfront cost. Here the annual benefit has two parts: the financial cash flow from energy savings or revenue, and a monetized carbon benefit representing avoided emissions valued at a carbon price. The two are summed and then discounted year by year by one plus r raised to the power of the year. Subtracting the initial investment gives green NPV; running the same calculation without the carbon term gives purely financial NPV, and the difference between them is the present value of the environmental component alone. The profitability index, present value of benefits divided by investment, complements NPV when capital is rationed, since it ranks projects by value created per dollar committed rather than by absolute size, with values above 1.0 clearing the hurdle. Discount rate choice dominates long-lived projects: at 6 percent a benefit arriving in year 20 is worth about 31 percent of its nominal amount, while at 3 percent it retains about 55 percent. The credibility of the carbon term rests entirely on the price applied, which might be an internal shadow price of 40 to 100 dollars per tonne, a market allowance price, or a social cost of carbon estimate. Mixing a private discount rate with a social damage value is a common inconsistency worth disclosing in any published appraisal.

The present value rule is Irving Fisher's. In The Rate of Interest of 1907 and again in The Theory of Interest of 1930 he argued that an asset is worth the discounted sum of the income it will produce, and that investment should proceed whenever present value exceeds cost. Joel Dean's Capital Budgeting of 1951 carried the rule into corporate practice, and the profitability index emerged from the capital rationing literature of the mid-1950s, notably work by James Lorie and Leonard Savage at the University of Chicago on selecting among projects under a fixed budget. Attaching a monetary value to emissions inside the same framework came from public sector appraisal rather than from finance. The UK Treasury's Green Book, which has guided British public investment appraisal for decades, sets a declining schedule of social discount rates beginning at 3.5 percent, and supplementary guidance requires departments to value greenhouse gas emissions using official carbon values. The 2006 Stern Review sharpened the debate by adopting a very low pure rate of time preference, which pushed the present value of distant climate damages far above conventional estimates. Corporate uptake followed disclosure pressure, with the Task Force on Climate-related Financial Disclosures issuing its recommendations in 2017.