Energy from primary energy sources is often transformed into different forms to make it easier to use, transport, or store. These forms are called energy currencies. Several authors have introduced the idea of energy currency as a way to think about these useful intermediate forms of energy.
Humans harvest energy from primary energy sources and eventually turn this energy into an energy service. The primary energy are the primary energy flows (like solar power) or primary fuels (like raw natural gas) that harvests energy from nature. The energy service is what maintains a high energy society (through heating, transportation, etc.). These intermediate forms, energy currencies, are used because they are flexible and transportable energy, which allows moving the energy supply almost anywhere. For example, while it's nice to enjoy sunlight, it doesn't recharge the battery in a cell phone; some energy conversion technology is required to get it into the specific form (and then moved with energy distribution technology, like pipelines or the electrical grid). The form that holds that energy from the energy conversion technology is sometimes referred to as an energy currency. There is always some energy loss when making an energy currency, often due to the second law of thermodynamics.
This model of energy currency as an intermediate step works best with electricity. Electricity is flexible, easy to use for just about anything, and is made specifically for the transportation and use of energy. Electricity will never run out because the electrons aren't getting used up; they're just carrying energy by moving. More energy can always be put into the same electrons over and over again, and taken out to use for more energy services.
Many people are talking about using molecular hydrogen as an energy currency (that's hydrogen as a molecule, rather than hydrogen as an atom), with the thought that hydrogen will become an increasingly important energy currency (and some disagree). Electricity, gasoline, and hydrogen are derived from primary energy and are easier to turn into useful work than most primary energy is. For example, a lump of coal can be burned for lighting a house, but it’s much cleaner (at least for the living room) to burn that lump of coal in a power plant and send the electricity through the grid to the house.
Energy currencies are flexible; they can be used for many energy services. For example, electricity is far more flexible than wind power. When an electric kettle boils water, the water boils no differently whether the electricity came from coal, natural gas, nuclear, biomass, wind, or hydro. The electricity could have also been used to play a computer game. See more in the article electricity as an energy currency.
This flexibility of use means that, in general, the amount of energy currency that the world uses is growing faster than primary energy use is. In other words, the percentage of energy use that goes to electricity is rising (electricity use is rising faster than energy use).
Gasoline and other secondary fuels are also often considered energy currencies. While these are not as flexible as electricity, they are an intermediate form of energy that is produced specifically to enable easy use of the stored chemical energy.
The idea of an energy currency gets far more murky with natural gas. Since natural gas can be used with very minimal processing it's often considered a primary energy source, but since it's delivering energy services it could be considered an energy currency. Natural gas is quite flexible and is used for (among many possibilities): electricity generation, home heating, and cooking. This flexibility is certainly greater than gasoline, but since it's not an intermediate source the idea of energy currency gets less clear.
Even more murky is the idea of nuclear fuel. This is an intermediate form of energy, put into this form in order to make it easier to use. Granted, this form is quite inflexible; nuclear fuel usually has to be put into a specific type of nuclear reactor, and not just any reactor will do! This means that while an intermediate form, it's not usually considered an energy currency.
Worldwide, more and more energy is being turned into energy currencies because of how useful they are. Please see the article on trends in energy currencies.