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

Of course all things aim to be 100% efficient, but  thats virtually impossible. A Sankey diagram is a good way to visualise how much energy is wasted/useful (the more useful energy that goes out, the more efficient the object is). For example...

The useful energy output for a lightbulb is light energy, because thats what we want to come out. However, some of the electrical energy is transferred into heat energy. This is an example of a very inefficient lightbulb, only 10% of the energy input comes out as useful energy, the rest comes out as heat (wasted) energy.

NOTE: Inefficiency is the same for many everyday situation, e.g. a fire (for warmth) creates light; a pepper grinder creates sound (even though you just want it to move).

NOTE NOTE: In Sankey diagrams, the 'down' arrow (s) is the wasted energy, the 'straight' arrow(s) is the useful energy