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Doodle Science teaches you high school physics in a less boring way in almost no time!
Different types of energy can be transferred from one type to another. You can see that a car engine transfers chemical energy, which is stored in the fuel, into kinetic energy in the engine and wheels so it can get you from A to B. In a lamp the electrical input is converted into light energy. However not all the energy put in is turned into useful energy, some of it is turned into energy we don't need such as heat.
Sankey diagrams summarise all the energy transfers taking place in a process. The thicker the arrow, the greater the amount of energy involved. This Sankey diagram for an electric lamp shows that most of the electrical energy is transferred as heat rather than light. This means it's very inefficient. We can calculate efficiency using this simple formula, useful energy, in this case 10J divided by the total energy input, which is 100J, which gives us an efficiency of 0.1 or 10% if you times it by 100.