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.
This one may seem simple, but that’s only because it is. If you were to move your savings from a traditional, brick-and-mortar bank with a low-interest rate and into a high-yield savings account online, over time you can make a surprising amount of extra cash. Online banks are FDIC-insured just like the traditional brick-and-mortar institutions, so your money is just as safe.

For example, you can design digital products, like animal clipart or a downloadable wedding poem that could be printed by the customer. Your design could be resold thousands of times without needing to create each item or ship it. Once you create your digital product and list it on a site like Etsy or Ebay, the revenue flows in with little oversight.
Greg Johnson is a personal finance and frugal travel expert who leveraged his online business to quit his 9-5 job, spend more time with his family, and travel the world. With his wife Holly, Greg co-owns two websites – Club Thrifty and Travel Blue Book. The couple has also co-authored a book, Zero Down Your Debt: Reclaim Your Income and Build a Life You'll Love. Find him on Instagram, Facebook, and Twitter @ClubThrifty.
When done correctly, investing can be a great way to generate residual income. There are many different types of investments you can choose from to earn income passively — whether you choose to purchase high dividend stocks, try peer to peer lending, or choose to invest in real estate. No matter what you choose to do, make sure you do your research first and talk to a tax advisor to ensure you understand your specific situation and what option is best for you.

One thing I’ve realized is this: It’s FAR easier to work for an employer than it is to develop durable passive income streams for the average person. Why? Because working for an employer in a place that “needs” you means that it’s possible to show up and give a 50% effort. You can show up, put in your time, go home, have a beer, watch TV, and rinse and repeat all without REALLY having to put in the effort.

The second category of passive income is drawing on sources that do not require capital to start, maintain, and grow. These are far better choices for those who want to start out on their own and build a fortune from nothing. They include assets you can create, such as a book, song, patent, trademark, Internet site, recurring commissions, or businesses that earn nearly infinite returns on equity such as a drop-ship e-commerce retailer that has little or no money tied up in operations but still turns a profit.
In the residual income model, the intrinsic value of a share of common stock is the sum of book value per share and the present value of expected future pershare residual income. In the residual income model, the equivalent mathematical expressions for intrinsic value of a common stock are V0=B0+∑t=1∞RIt(1+r)t=B0+∑t=1∞Et−rBt−1(1+r)t=B0+∑t=1∞(ROEt−r)Bt−1(1+r)t 
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.