What is net present value?
NPV discounts a stream of future cash flows back to today at a chosen rate, then subtracts the initial investment. A positive NPV means the project earns more than your discount rate; a negative NPV means it earns less.
Choosing a discount rate
Use your cost of capital — the return available on a comparable-risk alternative. Higher rates penalize distant cash flows more, which is why long-dated projects are so sensitive to the rate you pick.
NPV vs. IRR
NPV gives you a dollar value at a fixed rate; IRR gives you the rate at which NPV is zero. They usually agree, but NPV is the more reliable decision rule when comparing projects of different sizes.