Primary energy refers to the energy available as it exists in natural resources within the environment. These are unprocessed and unexploited energy products. Globally, the profile of primary energy consumption and origin is as follows:

We see here that 80% of primary energy consumed is from fossil sources (coal, oil, and natural gas). These originate from the fossilization of biomass, a process that takes millions of years. These energy sources have the advantage of being very dense, available in large quantities, easy to extract, transport, store, and thus cheap, and usable on demand.

To give an idea of the energy density of these sources, a 20-liter jerry can of gasoline represents the equivalent of 200 kWh of energy. It would take two months of operation of a 1 kW wind turbine to produce the same amount of energy (and the wind isn’t always blowing).

These energy sources come with major disadvantages. Firstly, they are non-renewable (or at least not quickly renewable, as it takes millions of years to fossilize biomass). They can therefore be considered stock resources, available in limited quantities and only in certain parts of the globe. Furthermore, their combustion (the way we transform their chemical energy into heat energy) emits large amounts of greenhouse gases, notably CO₂, which is responsible for global warming.

The rest of the primary energy consumed (approximately 20%) comes from renewable sources. They originate from resources that nature continually renews. They have the advantage of being available in unlimited quantities in the environment and, for the most part, are low in greenhouse gas emissions. Their major drawback is that they can be difficult to collect, are hard to store (except for biomass and hydropower), and some are intermittent, meaning they do not produce energy continuously or on demand.

So, what is the energy density between the two major families of energy sources? In 2019, humanity consumed 173,000 TWh of energy. One might wonder: is that a lot? Knowing that a standard solar panel produces 500 kWh of energy per year and the global population was 7.7 billion people, each person consumes an average of 22,367.5 kWh/year and would need 45 solar panels. This represents a surface area of 76.5 m², which is far from being available to each individual in some high-density regions. Furthermore, it is unlikely that the necessary materials would be available for such an area, making it a relatively expensive solution.

Converting 100% of the energy consumed into renewables presents a considerable technical challenge, as the technological maturity of these solutions lags behind fossil energy. This explains why a shift to 100% renewables, as desirable as it may be, is not feasible in the short term and represents the major challenge of the 21st century for humanity. Since the Industrial Revolution, humanity has significantly improved living conditions. Now, in the 21st century, the challenge is to sustain this system while making it accessible to an increasingly large population. This is what is known as the energy transition.