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.

You don't have to be Paul McCartney to rack up residuals. You might own properties that you rent out even though you're not a Realtor. Maybe a blog you started took off, and while you no longer work there, you still collect part of the profits. Or perhaps you're a serial entrepreneur who creates companies and moves on. You, too, are earning residual income.
Nowadays, anyone can create and sell t-shirts online, even with minimal design skill and coding knowledge. You can create designs on t-shirt marketplaces that do it all for you, upload designs to print-on-demand vendors who dropship orders for you, or set up an online store and sell tees you print and ship yourself, which can be the most profitable option.
The first form of residual income, the leftover cash after bills and expenses are paid, is important when a person is growing their savings account or is seeking a loan or financing. When getting a loan, there must be an adequate amount of residual income available to ensure a person has the money to make the loan payments each month. If there is not adequate residual income, financing or loans will not likely be approved.
To create residual income, you need to create something that people will continue to buy on a regular basis long after you’ve created it. A house is a prime example of this as people will continue to pay rent for the right to live in the house. A business needs to have products that are sold over and over again rather than trading the business owner’s time for money.