Three main statistical sources are used for the many measures of oil and other forms of energy cited in this book. These were British Petroleum's annual Statistical Review, published since 1959, with some oil data reaching back to 1938; the United Nations Yearbook of World Energy, with data for all forms of energy back to 1950; and the OECD/IEA publications on Energy Balances, published since 1976, with data for OECD countries back to 1960. A few other sources are used for certain long-run historical series.

Since the book frequently uses parallel data series from these sources for historical reference, it has to be noted that they are not precisely comparable. Statistics from them differ considerably in details of coverage and definitions, notably as regards energy trade; and have until recently followed different conventions for the ‘energy equivalents’ that they apply to express the main commercial primary fuels and forms of electricity in common units of measurement. (Solid fuels, oil and other liquid fuels, natural gases, and electricity from hydropower, geothermal, tidal or wind resources are primary energy. Electricity generated from fossil or nuclear fuels in power stations is secondary energy. Fuelwood and a wide range of other ‘vegetal fuels’ are generally considered ‘non-commercial’ and left out of national and world energy statistics.)

Energy equivalent values for the fossil fuels are fairly standardised, though BP uses averaged international values — i.e. one tonne of oil equals 1.5 tonnes of coal, 3 tonnes of lignite, and 1.111 cubic metres of natural gas – while the UN and IEA apply separate national averages for the fuels in each country.