Because a single BTU is so small, energy is usually measured in thousands or millions of BTU. For entire economies, energy is measured in quadrillion BTU, or "quads" for short. A quadrillion is equal to 10^15. In 2002, total US energy consumption was 97.4 quads.
The metric equivalent of the BTU is the Joule. One quad equals approximately 1.055 Exajoules (10^18 Joules).