When we talk about efficiency in terms of power sources, what we are really trying to determine is the ratio of the useful power output divided by the total electrical power consumed, which refers to both energy and heat. A big talking point in the debate of switching to renewable energies concerns their efficiency - yes they may be friendlier for the environment, but are they efficient? Below is a brief list of power sources annotated with their percentage efficiency. To learn more about the specific energy sources, watch the video!
Fossil fuel: 33%-45%
Nuclear: low 30's%-48%
Of course the efficiency of a certain power source involves a lot of factors, which is why there is a range of percentages. Every efficiency will depend not only on the quality of the machine generating the energy, but in the case of renewable energy, also the external environment, such as wind speed or hours of sun! In coal plants, for example, the plants can increase their efficiency by looping the emitted heat back into the generation station, thus reusing that heat to produce electricity, instead of just letting it escape into the atmosphere.