In 2008, total worldwide energy consumption was 474 exajoules (474×1018 J) with 80 to 90 percent derived from the combustion of fossil fuels. This is equivalent to an average power consumption rate of 15 terawatts (1.504×1013 W). 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's 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; for example, 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 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 m−2 in early January to 1321 W m−2 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 total energy rate is 174 petawatts (1.740×1017 W), plus or minus 3.5%. This value is the total rate of solar energy received by the planet; about half, 89 PW, reaches the Earth's surface.
The estimates of remaining non-renewable 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.
The United States Energy Information Administration regularly publishes a report on world consumption for most types of primary energy resources.
|Fuel type||Average power in TW|
solar energy, wood
The twentieth century saw a rapid twentyfold increase in the use of fossil fuels. Between 1980 and 2006, the worldwide annual growth rate was 2%. According to the US Energy Information Administration's 2006 estimate, the estimated 471.8 EJ total consumption in 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, natural gas, and nuclear became the fuels of choice for electricity generation and conservation measures increased energy efficiency. In the U.S. the average car more than doubled the number of miles per gallon. Japan, which bore the brunt of the oil shocks, made spectacular improvements and now has the highest energy efficiency in the world. From 1965 to 2008, the use of fossil fuels has continued to grow and their share of the energy supply has increased. From 2003 to 2008, coal, which is one of the dirtiest sources of energy, was the fastest growing fossil fuel..
In 2005 nuclear power accounted for 6.3% of world's total primary energy supply. The nuclear power production in 2006 accounted 2,658 TWh (23.3 EJ), 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.
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.
Worldwide hydroelectricity consumption reached 816 GW in 2005, consisting of 750 GW of large plants, and 66 GW of small hydro installations. Large hydro capacity totaling 10.9 GW was added by China, Brazil and India during the year, but there was a much faster growth (8%) in small hydro, with 5 GW added, mostly in China where some 58% of the world's small hydro plants are now located. In the Western world, although Canada is the largest producer of hydroelectricity in the world, the construction of large hydro plants has stagnated due to environmental concerns. The trend in both Canada and the United States has been to micro hydro because it has negligible environmental impacts and opens up many more locations for power generation. In British Columbia alone the estimates are that micro hydro will be able to more than double electricity production in the province.
Until the end of the nineteenth century biomass was the predominant fuel, today it has only a small share of the overall energy supply. Electricity produced from biomass sources was estimated at 44 GW for 2005. Biomass electricity generation increased by over 100% in Germany, Hungary, the Netherlands, Poland and Spain. A further 220 GW was used for heating (in 2004), bringing the total energy consumed from biomass to around 264 GW. The use of biomass fires for cooking is excluded.
World production of bioethanol increased by 8% in 2005 to reach 33 billion litres (8.72 billion US gallons), with most of the increase in the United States, bringing it level to the levels of consumption in Brazil. Biodiesel increased by 85% to 3.9 billion litres (1.03 billion US gallons), making it the fastest growing renewable energy source in 2005. Over 50% is produced in Germany.
According to the World Wind Energy Association, the installed capacity of wind power increased by 29 % from the end of 2007 to the end of 2008 to total 121 GW, with over half the increase in the United States, Spain and China. Doubling of capacity took about three years. The total installed capacity is approximately three to eight times that of the actual average power produced as the nominal capacity represents peak output; actual capacity is generally from 13-40% of the nominal capacity.
The available solar energy resources are 3.8 YJ/yr (120,000 TW). Less than 0.02% of available resources are sufficient to entirely replace fossil fuels and nuclear power as an energy source. Assuming that our rate of usage in 2005 remains constant, we will run out of conventional oil in 40 years (2045), coal in 154 yrs (2159). In practice neither will actually run out, as natural constraints will force production to decline as the remaining reserves dwindle.
In 2007 grid-connected photovoltaic electricity was the fastest growing energy source, with installations of all photovoltaics increasing by 83% in 2009 to bring the total installed capacity to 15 GW. Nearly half of the increase was in Germany, which is now the world's largest consumer of photovoltaic electricity (followed by Japan). Solar cell production increased by 50% in 2007, to 3,800 megawatts, and has been doubling every two years.
Geothermal energy is used commercially in over 70 countries. In the year 2004, 200 PJ (57 TWh) of electricity was generated from geothermal resources, and an additional 270 PJ of geothermal energy was used directly, mostly for space heating. In 2007, the world had a global capacity for 10 GW of electricity generation and an additional 28 GW of direct heating, including extraction by geothermal heat pumps. Heat pumps are small and widely distributed, so estimates of their total capacity are uncertain and range up to 100 GW. Heat pump capacity factors are low since demand is seasonal.
Energy consumption is loosely correlated with gross national product and climate, but there is a large difference even between the most highly developed countries, such as Japan and Germany with 6 kW per person and United States with 11.4 kW per person. In developing countries particularly those which are sub-tropical or tropical such as India the per person energy use is closer to 0.7 kW. Bangladesh has the lowest consumption with 0.2 kW per person.
The US consumes 25% of the world's energy with a share of global GDP at 22% and a share of the world population at 5%. The most significant growth of energy consumption is currently taking place in China, which has been growing at 5.5% per year over the last 25 years. Its population of 1.3 billion people (20% of the world population) is consuming energy at a rate of 1.6 kW per person.
Over the past four years, electricity consumption per capita in the U.S. has decreased about 1% per year between 2004 and 2008. Power consumption is projected to hit 4,333,631 million kilowatt hours by 2013, an annual growth rate of 1.93% for the next five years. Consumption increased from 3,715,949 in 2004 to an expected 3,937,879 million kilowatt hours per year in 2008, an increase of about 1.5% per year. The rate of increase has been steadily decreasing - it was 2.5% in the 1990s. U.S. population has been increasing about 1.3% per year, a total increase of about 6.7% over five years. The decrease has been mostly due to efficiency increases. Compact fluorescent bulbs, for example use about one third as much electricity as incandescents. LED bulbs, however, use about one tenth as much, and over their 50,000 to 100,000 hour lifetime are cheaper than compact fluorescents.
One metric of efficiency is energy intensity. This is a measure of the amount of energy it takes a country to produce a dollar of gross domestic product.
Industrial users (agriculture, mining, manufacturing, and construction) consume about 37% of the total 15 TW. Personal and commercial transportation consumes 20%; residential heating, lighting, and appliances use 11%; and commercial uses (lighting, heating and cooling of commercial buildings, and provision of water and sewer services) amount to 5% of the total. 
The other 27% of the world's energy is lost in energy transmission and generation. In 2005, global electricity consumption averaged 2 TW. The energy rate used to generate 2 TW of electricity is approximately 5 TW, as the efficiency of a typical existing power plant is around 38%. The new generation of gas-fired plants reaches a substantially higher efficiency of 55%. Coal is the most common fuel for the world's electricity plants.
Remaining reserves of fossil fuel are estimated as: Assessment Team |url= http://pubs.usgs.gov/dds/dds-060/ESpt4.html#Table | accessdate=2007-01-18}}</ref>
|Fuel||Energy reserves in ZJ|
Significant uncertainty exists for these numbers. The estimation of the remaining fossil fuels on the planet depends on a detailed understanding of the Earth crust. This understanding is still less than perfect. While modern drilling technology makes it possible to drill wells in up to 3 km of water to verify the exact composition of the geology, one half of the ocean is deeper than 3 km, leaving about a third of the planet beyond the reach of detailed analysis. The Energy Watch Group reports suggest that supplying the demand for oil may be insufficient, and that uranium resources would be exhausted within 70 years. However, these views are greatly at variance with those of most industry observers.
Coal is the most abundant fossil fuel. This was the fuel that launched the industrial revolution and has continued to grow in use; China, which already has many of the world's most polluted cities, was in 2007 building about two coal fired power plants every week. Coal is the fastest growing fossil fuel and its large reserves would make it a popular candidate to meet the energy demand of the global community, short of global warming concerns and other pollutants. According to the International Energy Agency the proven reserves of coal are around 909 billion tonnes, which could sustain the current production rate for 155 years, although at a 5% growth per annum this would be reduced to 45 years, or until 2051. With the Fischer-Tropsch process it is possible to make liquid fuels such as diesel and jet fuel from coal. In the United States, 49% of electricity generation comes from burning coal.
It is estimated that there may be 57 ZJ of oil reserves on Earth (although estimates vary from a low of 8 ZJ, consisting of currently proven and recoverable reserves, to a maximum of 110 ZJ) consisting of available, but not necessarily recoverable reserves, and including optimistic estimates for unconventional sources such as tar sands and oil shale. Current consensus among the 18 recognized estimates of supply profiles is that the peak of extraction will occur in 2020 at the rate of 93-million barrels per day (mbd). Current oil consumption is at the rate of 0.18 ZJ per year (31.1 billion barrels) or 85-mbd.
There is growing concern that peak oil production may be reached in the near future, resulting in severe oil price increases. A 2005 French Economics, Industry and Finance Ministry report suggested a worst-case scenario that could occur as early as 2013. There are also theories that peak of the global oil production may occur in as little as 2–3 years. The ASPO predicts peak year to be in 2010. Some other theories present the view that it has already taken place in 2005. World crude oil production (including lease condensates) according to US EIA data decreased from a peak of 73.720 mbd in 2005 to 73.437 in 2006, 72.981 in 2007, 73.697 in 2008. According to peak oil theory, increasing production will lead to a more rapid collapse of production in the future, while decreasing production will lead to a slower decrease, as the bell-shaped curve will be spread out over more years.
Political considerations over the security of supplies, environmental concerns related to global warming and sustainability are expected to move the world's energy consumption away from fossil fuels. The concept of peak oil shows that we have used about half of the available petroleum resources, and predicts a decrease of production.
A government led move away from fossil fuels would most likely create economic pressure through carbon emissions and green taxation. Some countries are taking action as a result of the Kyoto Protocol, and further steps in this direction are proposed. For example, the European Commission has proposed that the energy policy of the European Union should set a binding target of increasing the level of renewable energy in the EU's overall mix from less than 7% today to 20% by 2020.
The antithesis of sustainability is a disregard for limits, commonly referred to as the Easter Island Effect, which is the concept of being unable to develop sustainability, resulting in the depletion of natural resources.
The International Atomic Energy Agency estimates the remaining uranium resources to be equal to 2500 ZJ. This assumes the use of breeder reactors which are able to create more fissile material than they consume. IPCC estimated currently proved economically recoverable uranium deposits for once-through fuel cycles reactors to be only 2 ZJ. The ultimately recoverable uranium is estimated to be 17 ZJ for once-through reactors and 1000 ZJ with reprocessing and fast breeder reactors.
Resources and technology do not constrain the capacity of nuclear power to contribute to meeting the energy demand for the 21st century. However, political and environmental concerns about nuclear safety and radioactive waste started to limit the growth of this energy supply at the end of last century, particularly due to a number of nuclear accidents. Concerns about nuclear proliferation (especially with plutonium produced by breeder reactors) mean that the development of nuclear power by countries such as Iran and Syria is being actively discouraged by the international community.
Fusion power is the process driving our sun and other stars. It generates large quantities of heat by fusing the nuclei of hydrogen or helium isotopes, which may be derived from seawater. The heat can theoretically be harnessed to generate electricity. The temperatures and pressures needed to sustain fusion make it a very difficult process to control. The tantalizing potential of fusion is its theoretical ability to supply vast quantities of energy, with relatively little pollution. Although both the United States and the European Union, along with other countries, are supporting fusion research (such as investing in the ITER facility), according to one report, inadequate research has stalled progress in fusion research for the past 20 years.
Renewable resources are available each year, unlike non-renewable resources which are eventually depleted. A simple comparison is a coal mine and a forest. While the forest could be depleted, if it is managed properly it represents a continuous supply of energy, vs the coal mine which once it has been exhausted is gone. Most of earth's available energy resources are renewable resources. Renewable resources account for more than 93 percent of total U.S. energy reserves. Annual renewable resources were multiplied times thirty years for comparison with non-renewable resources. In other words, if all non-renewable resources were uniformly exhausted in 30 years, they would only account for 7 percent of available resources each year, if all available renewable resources were developed.
Renewable energy sources are even larger than the traditional fossil fuels and in theory can easily supply the world's energy needs. 89 PW of solar power falls on the planet's surface. While it is not possible to capture all, or even most, of this energy, capturing less than 0.02% would be enough to meet the current energy needs. Barriers to further solar generation include the high price of making solar cells and reliance on weather patterns to generate electricity. Also, solar generation does not produce electricity at night, which is a particular problem in high northern and southern latitude countries; energy demand is highest in winter, while availability of solar energy is lowest. This could be overcome by buying power from countries closer to the equator during winter months. Globally, solar generation is the fastest growing source of energy, seeing an annual average growth of 35% over the past few years. Japan, Europe, China, U.S. and India are the major growing investors in solar energy. Advances in technology and economies of scale, along with demand for solutions to global warming, have led photovoltaics to become the most likely candidate to replace nuclear and fossil fuels.
The available wind energy estimates range from 300 TW to 870 TW. Using the lower estimate, just 5% of the available wind energy would supply the current worldwide energy needs. Most of this wind energy is available over the open ocean. The oceans cover 71% of the planet and wind tends to blow stronger over open water because there are fewer obstructions.
At the end of 2005, 0.3 GW of electricity was produced by tidal power. Due to the tidal forces created by the Moon (68%) and the Sun (32%), and the Earth's relative rotation with respect to Moon and Sun, there are fluctuating tides. These tidal fluctuations result in dissipation at an average rate of about 3.7 TW. As a result, the rotational speed of the Earth decreases, and the distance of the Moon to the Earth increases, on geological time scales. In several billion years, the Earth will rotate at the same speed as the Moon is revolving around it. So, several TW of tidal energy can be produced without having a significant effect on celestial mechanics.
Another physical limitation is the energy available in the tidal fluctuations of the oceans, which is about 0.6 EJ (exajoule). Note this is only a tiny fraction of the total rotational energy of the Earth. Without forcing, this energy would be dissipated (at a dissipation rate of 3.7 TW) in about four semi-diurnal tide periods. So, dissipation plays a significant role in the tidal dynamics of the oceans. Therefore, this limits the available tidal energy to around 0.8 TW (20% of the dissipation rate) in order not to disturb the tidal dynamics too much.
Waves are derived from wind, which is in turn derived from solar energy, and at each conversion there is a drop of about two orders of magnitude in available energy. The total power of waves that wash against our shores add up to 3 TW.
Estimates of exploitable worldwide geothermal energy resources vary considerably, depending on assumed investements in technology and exploration and guesses about geological formations. According to a 1999 study, it was thought that this might amount to between 65 and 138 GW of electrical generation capacity 'using enhanced technology'. Other estimates range from 35 to 2000 GW of electrical generation capacity, with a further potential for 140 EJ/year of direct use.
A 2006 report by MIT that took into account the use of Enhanced Geothermal Systems (EGS) concluded that it would be affordable to generate 100 GWe (gigawatts of electricity) or more by 2050, just in the United States, for a maximum investment of 1 billion US dollars in research and development over 15 years. The MIT report calculated the world's total EGS resources to be over 13 YJ, of which over 200 ZJ would be extractable, with the potential to increase this to over 2 YJ with technology improvements - sufficient to provide all the world's energy needs for several millennia. The total heat content of the Earth is 13,000,000 YJ.
Production of biomass and biofuels are growing industries as interest in sustainable fuel sources is growing. Utilizing waste products avoids a food vs fuel trade-off, and burning methane gas reduces greenhouse gas emissions, because even though it releases carbon dioxide, carbon dioxide is 23 times less of a greenhouse gas than is methane. Biofuels represent a sustainable partial replacement for fossil fuels, but their net impact on greenhouse gas emissions depends on the agricultural practices used to grow the plants used as feedstock to create the fuels. While it is widely believed that biofuels can be carbon-neutral, there is evidence that biofuels produced by current farming methods are substantial net carbon emitters. Geothermal and biomass are the only two renewable energy sources which require careful management to avoid local depletion.
In 2005, hydroelectric power supplied 16.4% of world electricity.
Denmark and Germany have started to make investments in solar energy, despite their unfavorable geographic locations. Germany is now the largest consumer of photovoltaic cells in the world. Denmark and Germany have installed 3 GW and 17 GW of wind power respectively. In 2005, wind generated 18.5% of all the electricity in Denmark. Brazil invests in ethanol production from sugar cane which is now a significant part of the transportation fuel in that country. Starting in 1965, France made large investments in nuclear power and to this date three quarters of its electricity comes from nuclear reactors. Switzerland is planning to cut its energy consumption by more than half to become a 2000-watt society by 2050 and the United Kingdom is working towards a zero energy building standard for all new housing by 2016. In 2005, the Swedish government announced the oil phase-out in Sweden with the intention to become the first country to break its dependence on fossil fuel by 2020.