That income is considered residual income because as long as the apartment is rented and the rent is collected, the income is earned without additional effort. The effort came when the property was purchased and a tenant was found. Each month after that, the money automatically is paid without buying the apartment again or finding the same tenant each month.
Residual income is different from a salary, or linear income, which is paid out strictly based on the number of hours a person works. Someone who works on a salary is often said to work “paycheck to paycheck.” This is because he pays all of his bills with his first paycheck and then must wait until he gets paid again to have more money. Ideally, someone will work hard building up a business so that he can enjoy the residual income once his goals have been met. Then he can work on additional projects while still earning money from his business.
Residual income is calculated as net income less a charge for the cost of capital. The charge is known as the equity charge and is calculated as the value of equity capital multiplied by the cost of equity or the required rate of return on equity. Given the opportunity cost of equity, a company can have positive net income but negative residual income.