While net present value (NPV) calculations are useful when you are valuing investment opportunities, the process is by no means perfect.

The biggest disadvantage to the calculation of NPV is its sensitivity to discount rates. After all, NPV computations are really just a summation of multiple discounted cash flows - both positive and negative - converted into present value terms for the same point in time (usually when the cash flows begin). As such, the discount rate used in the denominators of each present value (PV) computation is critical in determining what the final NPV number will turn out to be. A small increase or decrease in the discount rate will have a considerable effect on the final output.

Let's say you were trying to value an investment that would cost you $4,000 up front today, but was expected to pay you $1,000 in annual profits for five years (for a total nominal amount of $5,000), beginning at the end of this year. If you use a 5% discount rate in your NPV calculation, your five $1,000 payments are equal to $4,329.48 of today's dollars. Subtracting the $4,000 initial payment, you are left with an NPV of $329.28. (To learn more about calculating NPV, see Understanding The Time Value Of Money and Anything But Ordinary: Calculating The Present And Future Value Of Annuities.)

However, if you raise the discount rate from 5% to 10%, you get a very different NPV result. At a 10% discount rate, your investment's cash flows add up to a present value of $3,790.79. Subtract the $4,000 initial cost from this amount, and you're left with a negative NPV of $209.21. Simply by adjusting the rate, you have gone from having an investment that creates $329.28 of value to having one that destroys $209.21 instead.

Source : Investopedia

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