In order to directly compare world energy resources and consumption of energy, this article uses SI units and prefixes and measures energy rate (or power) in watts (W) and amounts of energy in joules (J). One watt is one joule per second.

In 2004, the total worldwide energy consumption of the human race was 500 EJ (= 5 x 1020 J) with 86.5% derived from the combustion of fossil fuels, although there is at least 10% uncertainty in that figure. This is equivalent to 15 TW (= 1.5 x 1013 W) of power. Not all of the world's economies track their energy consumption with the same rigor, and the exact energy content of a barrel of oil or a ton of coal will vary with quality.

Most of the world energy resources are from the sun's rays hitting earth - some of that energy has been preserved as fossil energy, some is directly or indirectly usable e.g. via wind, hydro or wave power. The term solar constant is the amount of incoming solar electromagnetic radiation per unit area, measured on the outer surface of Earth's atmosphere, in a plane perpendicular to the rays. The solar constant includes all types of solar radiation, not just the visible light. It is measured by satellite to be roughly 1366 watts per square meter, though it fluctuates by about 6.9% during a year - from 1412 W/m2 in early January to 1321 W/m2 in early July, due to the earth's varying distance from the sun, and by a few parts per thousand from day to day. For the whole Earth, with a cross section of 127,400,000 km², the power is 1.740×1017 W, plus or minus 3.5%.

The estimates of remaining worldwide energy resources vary, with the remaining fossil fuels totaling an estimated 0.4 YJ (1 YJ = 1024J) and the available nuclear fuel such as uranium exceeding 2.5 YJ. Fossil fuels range from 0.6-3 YJ if estimates of reserves of methane clathrates are accurate and become technically extractable. Mostly thanks to the Sun, the world also has a renewable usable energy flux that exceeds 120 PW (8,000 times 2004 total usage), or 3.8 YJ/yr, dwarfing all non-renewable resources.

Fossil Fuel Energy

Since the advent of the industrial revolution, the worldwide energy consumption has been growing steadily. In 1890 the consumption of fossil fuels roughly equaled the amount of biomass fuel burned by households and industry. In 1900, global energy consumption equaled 0.7 TW (0.7×1012 watts).

The twentieth century saw a rapid twentyfold increase in the use of fossil fuels. Between 1980 and 2004, the worldwide annual growth rate was 2%. According to the US Energy Information Administration's 2006 estimate, the estimated 15TW total energy consumption of 2004 was divided as follows, with fossil fuels supplying 86% of the world's energy.

Coal fueled the industrial revolution in the 18th and 19th century. With the advent of the automobile, airplanes and the spreading use of electricity, oil became the dominant fuel during the twentieth century. The growth of oil as the largest fossil fuel was further enabled by steadily dropping prices from 1920 until 1973. After the oil shocks of 1973 and 1979, during which the price of oil increased from 5 to 45 US dollars per barrel, there was a shift away from oil. Coal and nuclear became the fuels of choice for electricity generation and conservation measures increased energy efficiency. In the US the average car more than doubled the number of miles per gallon. Japan, who bore the brunt of the oil shocks, made spectacular improvements and now has the highest energy efficiency in the world. Over the last forty years, the use of fossil fuels has continued to grow and their share of the energy supply has increased. In the last three years, coal, which is one of the dirtiest sources of energy, has become the fastest growing fossil fuel. Photovoltaics are rapidly becoming available to replace fossil fuels as the dominant energy source[citation needed]. Note the earlier comparison of availability: The total resources of all fossil fuels amount to about 0.4 YJ total, while the availability of solar power is 3.8 YJ per year.

Nuclear Energy

In 2005 nuclear energy accounted 6.3% of world's total primary energy supply. The nuclear power production in 2006 accounted 2,658 TWh, which was 16% of world's total electricity production. In November 2007, there were 439 operational nuclear reactors worldwide, with total capacity of 372,002 MWe. A further 33 reactors were under construction, 94 reactors were planned and 222 reactors were proposed. Among the nations not currently using nuclear power, 25 countries are building them, or are proposing to do so. A few nations have announced plans to phase out nuclear power altogether, but to date only Italy has done so (though Italy continues to import electricity from nations with active nuclear power plants). In addition, while Austria, the Philippines and North Korea have built nuclear power stations, these nations abandoned them before they could be fueled and operated.

Renewable Energy

In 2004, renewable energy supplied around 7% of the world's energy consumption. The renewables sector has been growing significantly since the last years of the 20th century, and in 2005 the total new investment was estimated to have been 38 billion US dollars. Germany and China lead with investments of about 7 billion US dollars each, followed by the United States, Spain, Japan, and India. This resulted in an additional 35 GW of capacity during the year.

http://en.wikipedia.org/wiki/Energy_use