Power Tab can be found at the top of the window when you press E facing any machine or crafting station that requires fuel. It displays the power requirements for the machine or crafting station to function properly.
There are several forms of power that can be used, organised into different categories. One of the more common, especially earlier on, is fuel supply. Fuel supply requires a fuel source in order to function. In this category, the left-most slot represents fuel being used, where the two slots to the right represent the backlog of fuel to be burned. From the backlog, the left-most slot will be drawn from first. The backlog is shown in linked storage, which allows for the backlog to be the first store location for produced charcoal.
Mechanical power and the power grid are other categories within the power tab. They are generated with remote blocks specialised for generating mechanical energy and electricity respectively. Mechanical power and electrical power are discussed in more detail on their respective pages.