The weighted average cost of capital (WACC) is simply an average cost of the two types of capital: debt and equity. It tells us the amount an investor would need in compensation to invest in a project. Therefore if we were to offer a lower return than the WACC, we would find that we have no investors; a return greater than WACC would lead to a situation where we have excess demand for our project.
To theoretically calculate the rate of return on a project we would need to compare it with existing returns on projects which have similar risk characteristics, and then set the return similar to these projects to ensure that we can attract finance. [...]