We use energy in several different forms as we go about our daily lives, and rarely stop to think about the consequences of doing so. Energy is needed not only for our domestic needs, but also to fuel our factories and provide the motive power for transportation, whether that is by road, rail, air, or sea. The total world energy consumption in 2002 was just over 400 × 1015 Btu (British thermal units), or 10 Gtoe (Billion tonnes of oil equivalent). The distribution of that demand, by economic sector, is shown in Figure 4.1. Although the distribution by economic sector varies widely from country to country, depending largely on the degree of industrialization, overall approximately 25% of all energy is used to provide transportation, 32% is used to fuel industrial operations, while the balance is used for a range of activities, including the heating of both public and private buildings. A small quantity of primary energy resources are also used for so-called “non-energy” uses, such as chemical feedstocks used to produce plastics.

Although it is difficult to obtain a more detailed breakdown of energy consumption by economic sector on a global basis than that shown in Figure 4.1, most industrialized countries provide separate data for both the commercial and residential sectors. Energy demand by economic sector for the USA in 2000, for example, is shown in Figure 4.2.