According to the narrator, it takes a lump of coal the size of a \textbf{tennis ball} in order to make a single cup of coffee.
This comparison helps to visualize the amount of coal needed to produce the energy required to make just one cup of coffee. Coal is a source of energy that is used in power plants to generate electricity, and it is often measured in terms of the amount needed for different tasks, such as making coffee.
By using the size of a tennis ball as a reference point, the narrator is emphasizing the small amount of coal needed to produce the energy for a cup of coffee, highlighting the efficiency and power of coal as a fuel source.
