Opportunity cost is the basic concept at the heart of residual income. Opportunity cost refers to what you are giving up to use an asset for a particular project or investment. Let's say you start with $100,000 cash in your stock portfolio and grew that money to $104,000 by spending only a few hours per month trading stocks. Now $4,000 in profit may look just fine when you consider that you only worked, say, 20 hours for it in total. However, once you consider your opportunity cost, your little hobby will not look nearly as profitable. If banks are paying six percent for risk-free certificates of deposit, your $100,000 would have grown to $106,000 simply by depositing it in a bank. Your opportunity cost is $6,000

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.