Secondary energy sources, also called energy carriers, are derived from the transformation of primary energy sources. They are called energy carriers, because they move energy in a useable form from one place to another. The well-known energy carriers are:
Electricity and hydrogen made from primary energy sources such as coal, natural gas, nuclear energy, petroleum, and renewable energy sources. Electricity is particularly useful since it has low entropy (is highly ordered) and can be converted into other forms of energy very efficiently. Simply, we cannot say that hydrogen have potential to offset fossil fuels.
Secondary energy sources are used, because its using is easier than using a primary energy source. For example, using electricity for lighting is safer than using petroleum in candles or kerosene lamps.
On the other hand any conversion of primary energy to energy carrier is associated with some inefficiency. Therefore when dealing with secondary energy source, we have to always consider the way, how the carrier was made.