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).
To make this a bit less abstract, 45 million tons of coal would be a pile 10 feet thick, one mile wide and about 3.3 miles long. At 60 mph, it would take about 9 minutes to drive around the pile.
In terms of electricity, the energy content of 1 quad is equal to about 293 terawatt-hours or 33 gigawatt-years. However, a typical steam-turbine power plant burning fossil fuels is only able to capture about a third of the energy in the fuel, so 1 quad of fuel actually produces about 11 gigwatt-years of electricity.