Writing an e-book is very popular among bloggers, as many have noted that “it's just a bunch of blog posts put together!” You will not only have to make an investment of time and energy to create the e-book, but market it correctly. However, if marketed correctly (through blogging affiliates in your niche, for example), you could have residual sales that last a very long time.
However, with passive income, there is not a direct connection to time involved. Once the original work is completed, the income continues to come in as long as demand for the product or service exists. Each time a song is downloaded, the musician receives money as a passive income. They did not have to record the song again or do additional work for each download, yet they are paid for their original work.
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.