8-10 percent is a common assumption for cost of capital. My preference is to calculate the IRR as a quick check to see if the investment makes sense at first blush. If you do this, always do a quick check of undiscounted cash flow also, since value acceleration projects flip the interpretation of IRR. For that matter, my first step is always to calculate back-of-the-envelope undiscounted cash flow to see if I am even in the ballgame.
I wouldn't worry too much about your company's WACC. The real key is understanding your company's hurdle rates for its various types of investments.
Most likely the funding for a project will come out of an exisitng budget for capital investments or an operating budget. With budget constraints or an annual business plan your project must trump other projects more than it must trump a financing cost of capital.
Different project types may have different criteria they must meet.
Quick-hit expense projects may have hurdles like time to payout, while more significant capital projects may have hurdles such as IRR or PVR (PVR is value generated per dollar invested, in a rough sense).
Just ask around about where you will get the funding for the project, and the person or group that controls the purse strings will tell you how good your project needs to be and what you need to do to get the money.
ETA: But it is still good to ask around at your company about the appropriate discount rate to use for evaluation, which may resemble the company's WACC or even marginal cost of capital. There may even be a different discount rate to use for different types of projects in terms of risk or strategic/budget "bucket" (which I really dislike in most cases).
This post was edited on 4/4 at 12:28 am