4.5 describe a variety of everyday and scientific devices and situations, explaining the fate of the input energy in terms of the above relationship, including their representation by Sankey diagrams

With all devices that aim to use energy for a reason, some of the energy put in to run it comes out as a non useful form of energy. The more energy that comes out as useful, the more efficient the object is. For instance, a light bulb wants to create light energy, but it creates heat at the same time. This is the same for many processes: a fire (for warmth) creates light; a pepper grinder creates sound (even though you just want it to move).

Sankey diagrams use an arrow to represent the energy going in and out of a process:

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Here 100j of energy is going in. All 100j must come out. 90j come out as heat, 10j come out as light.