The present value of a stream of income or expenses is found the following process.
First, a discount rate, r, is selected that reflects the opportunity cost of funds involved. For example, a risk free discount rate may be equal to the current rate of Treasury securities, about 2.5% to 3% currently. A discount rate that reflects greater risk, such as a Baa bond rate, is about 5.25%.
Next, the future income or expenditures flows are determined. For example, if you decide to invest $1000 in a project that is expected to pay out the following amounts:
End of Year 1: $300
End of Year 2: $500
End of Year 3: $400
And the discount rate is 3%, the present value of the future flows is:
PV = $300/1.03 + $500/(1.032) + $400/(1.033) = $291.26 + $471.30 + $366.06 = $1128.62
The future stream of income from the project is equal to $1128.62 invested at 3% interest rate (the discount rate).
Net present value, NPV, is equal to the net of future revenues and costs. Since the cost of buying the investment is $1000 today, the net present value is equal to
NPV = -$1000 + $1128.62 = $128.62
Or this investment pays more than an equivalent investment paying 3% interest.
Note that for an investment to be a “good choice,” the NPV has to be positive or the present value of current and future earnings or income has to exceed the present value of current and future expenses