Since energy can neither be created nor destroyed, the total energy in the environment is essentially constant – there can never be an energy crisis. What is required is energy in a variety of useful forms together with a range of different energy converters. A source of useful energy is called a fuel. It is both the amounts of fuels that are available, and the efficiency of conversion of these fuels, that has led to the term ‘energy crisis’ – it should more accurately be described as a ‘fuel crisis’.