Question: How much energy does the US use per year?
About the article "United States", section "Energy".

Candidate passages:
Primary energy use in the United States was 25,155 TWh and 82 TWh per million persons in 2009.
Primary energy use was 1,100 TWh less in the US than in China in 2009.
Same year the share of energy import in the US was 26% of the primary energy use.
The energy import declined ca 22% and the annual CO2 emissions ca 10% in 2009 compared to 2004. 
Artificial lighting consumes a significant part of all electrical energy consumed worldwide.
In homes and offices from 20 to 50 percent of total energy consumed is due to lighting.
Most importantly, for some buildings over 90 percent of lighting energy consumed can be an unnecessary expense through over-illumination.
The cost of that lighting can be substantial.
A single 100 W light bulb used just 6 hours a day can cost over $25 per year to use (.12/kWh).
According to the UN Environment Programme/Global Environment Facility en.lighten initiative, by simply replacing all incandescent lamps with energy efficient compact fluorescent lamps globally, 409 TWh per year would be saved, which is approximately 2.5% of global electricity consumption.
This is equivalent to the combined yearly electricity consumption of the United Kingdom and Denmark.
Thus lighting represents a critical component of energy use today, especially in large office buildings where there are many alternatives for energy usage in lighting.
There are several strategies available to minimize energy requirements in any building:  Specification of illumination requirements for each given use area.
Analysis of lighting quality to ensure that adverse components of lighting (for example, glare or incorrect color spectrum) are not biasing the design.
Integration of space planning and interior architecture (including choice of interior surfaces and room geometries) to lighting design.
Design of time of day use that does not expend unnecessary energy.
Selection of fixture and lamp types that reflect best available technology for energy conservation.
Training of building occupants to use lighting equipment in most efficient manner.
Maintenance of lighting systems to minimize energy wastage.
Use of natural light — some big box stores are being built (ca 2006 on) with numerous plastic bubble skylights, in many cases completely obviating the need for interior artificial lighting for many hours of the day.
Load shedding can help reduce the power requested by individuals to the main power supply.
Load shedding can be done on an individual level, at a building level, or even at a regional level.
Specification of illumination requirements is the basic concept of deciding how much illumination is required for a given task.
Clearly, much less light is required to illuminate a hallway or bathroom compared to that needed for a word processing work station.
Generally speaking, the energy expended is proportional to the design illumination level.
For example, a lighting level of 80 footcandles might be chosen for a work environment involving meeting rooms and conferences, whereas a level of 40 footcandles could be selected for building hallways.
If the hallway standard simply emulates the conference room needs, then twice the amount of energy will be consumed as is needed for hallways.
Unfortunately, most of the lighting standards even today have been specified by industrial groups who manufacture and sell lighting, so that a historical commercial bias exists in designing most building lighting, especially for office and industrial settings. 
In physics, energy is a fundamental property of any physical object or system of objects which is used to describe and predict its interactions with other objects.
Although any energy in any single form can be transformed into another form, the law of conservation of energy states that the total energy of a system can only change if energy is transferred into or out of the system.
This means that it is impossible to create or destroy energy.
The total energy of a system can be calculated by adding up all forms of energy in the system.
Common energy forms include the kinetic energy of a moving object, the radiant energy carried by light and other electromagnetic radiation, and various types of potential energy such as gravitational and elastic.
Energy is measured in SI units of joules ( J ).
Common types of energy transfer and transformation include processes such as heating a material, performing mechanical work on an object, generating or making use of electric energy, and many chemical reactions.
Units of measurement for energy are usually defined via a work process.
The work performed by a given body on another is defined in physics as the force (SI unit: newton) applied by the given body, multiplied by the distance (SI unit: metre) of movement against the opposing force exerted by the other body.
Thus, the energy unit is the newton-metre, which is called the joule.
The SI unit of power (energy per unit time) is the watt, which is simply a joule per second.
Thus, a joule is a watt-second, so 3600 joules equal a watt-hour.
The CGS energy unit is the erg, and the imperial and US customary unit is the foot pound.
Other energy units such as the electron volt, food calorie or thermodynamic kcal (based on the temperature change of water in a heating process), and BTU are used in specific areas of science and commerce and have unit conversion factors relating them to the joule.
Not all of the energy in a system can be transformed or transferred by a work process; the amount that can is called the available energy.
In particular the second law of thermodynamics limits the amount thermal energy that can be transformed into other forms of energy.
Mechanical and other forms of energy can be transformed in the other direction into thermal energy without such limitations.
Potential energy is energy stored by virtue of the position of an object in a force field, such as a gravitational, electric or magnetic field.
For example, lifting an object against gravity performs work on the object and stores gravitational potential energy; if it falls, gravity does work on the object which transforms the potential energy to kinetic energy associated with its speed.
Some specific forms of energy include elastic energy due to the stretching or deformation of solid objects, chemical energy such as is released when a fuel burns, and thermal energy, the microscopic kinetic and potential energies of the disordered motions of the particles making up matter.
Any object that has mass when stationary (thus called rest mass), equivalently has rest energy as can be calculated using Albert Einstein's equation "E" = "mc"2.
Being a form of energy, rest energy can be transformed to or from "other forms" of energy, while the total amount of energy does not change.
From this perspective, the amount of matter in the universe contributes to its total energy.
Similarly, all energy manifests as a proportionate amount of mass.
For example, adding 25 kilowatt-hours (90 megajoules) of "any form" of energy to an object increases its mass by 1 microgram.
If you had a sensitive enough mass balance or scale, this mass increase could be measured.
Our Sun (or a nuclear bomb) transforms nuclear potential energy to other forms of energy; its total mass does not decrease due to that in itself (since it still contains the same total energy even if in different forms), but its mass does decrease when the energy escapes out to its surroundings, largely as radiant energy.
A new form of energy can not be defined arbitrarily.
In order to be valid, it must be shown to be transformable to or from a predictable amount of some known form(s) of energy, thus showing how much energy it represents in the same units used for all other forms.
It must obey conservation of energy, so it must never decrease or increase except via such a transformation (or transfer).
Also, if a proposed new form of energy can be shown not to change the mass of a system in proportion to its energy, then it is not a form of energy.
Living organisms require available energy to stay alive; humans get such energy from food along with the oxygen needed to metabolize it.
Civilization requires a supply of energy to function; energy resources such as fossil fuels are a vital topic in economics and politics.
Earth's climate and ecosystem are driven by the radiant energy Earth receives from the sun (as well as the geothermal energy contained within the earth), and are sensitive to changes in the amount received.
The word "energy" is also used outside of physics in many ways, which can lead to ambiguity and inconsistency.
The vernacular terminology is not consistent with technical terminology.
For example, while energy is always conserved (in the sense that the total energy does not change despite energy transformations), energy can be converted into a form, e.g., thermal energy, that cannot be utilized to perform work.
When one talks about "conserving energy by driving less", one talks about conserving fossil fuels and preventing useful energy from being lost as heat.
This usage of "conserve" differs from that of the law of conservation of energy. 
Sustainable Silicon Valley (SSV) chose CO2 emissions as a benchmark because it encompasses both energy use and its environmental consequences in a single measure.
It reflects improvement in energy efficiency and transitions from more to less polluting fuels.
CO2 is the largest contributor to human-generated greenhouse gases.
Recognizing the importance of the climate change issue and the area's drive to remain competitive in a location with high energy costs, SSV organized a regional voluntary initiative that has a visionary target of reducing CO2 emissions by 20% below the region's 1990 levels by the year 2010.
Partners in SSV choose their own baseline year and a CO2 percentage reduction goal to reach by 2010.
Each pledging partner also chooses how to meet this target, whether by equipment efficiency improvements (e.g.
new equipment), conserving energy (e.g.
behavioral changes), increasing the use of renewable energy sources (e.g.
photovoltaic systems), purchasing green power and/or promoting alternative commute options and supporting the purchase of low emission vehicles.
Most human-caused CO2 results from the burning of fossil fuels such as gasoline, diesel and natural gas.
In addition to burning fossil fuels directly in cars and appliances like home heating systems, people create CO2 emissions by using electricity, most of which in the United States is generated by burning fossil fuels.
SSV pledging partners address this issue though increased energy efficiency and increased use of renewable energy sources.
Pledging organizations have enjoyed impressive cost savings through their participation in SSV.
The climate change resulting from the accumulation of human-generated greenhouse gases like CO2 is one of the biggest environmental challenges of our time.
The consequences of climate change are far-reaching and will affect all of us on many levels.
How does energy efficiency relate to the SSV's CO2 priority?Renewable and other energy use, energy efficiency, and the resulting CO2 emissions, were chosen by the group as a key action opportunity, because:Many Silicon Valley organizations had demonstrated significant energy savings, in part from the recent energy crisis, resulting in cost savings and environmental performance improvements.
Other regions and organizations (Sonoma County, the City of Toronto, various Silicon Valley businesses) had already ‘signed up' to specific CO2 emissions reductions goals.
The goal selected by the group was consistent with other existing regional emissions reduction goals and represents a stretch goal for the region.
Reducing energy intensity (energy used per unit output) across the commercial, residential and transportation sectors was seen as having multiple benefits, including (but not limited to): opportunities for cost savings, success through broad participation and collaboration, reduction of the environmental impact of the region, and the demonstration of a leadership role on an issue of growing policy and economic significance. 
 in energy storage and in fuel.
In energy storage applications the energy density relates the mass of an energy store to the volume of the storage facility, e.g.
the fuel tank.
The higher the energy density of the fuel, the more energy may be stored or transported for the same amount of volume.
The energy density of a fuel per unit mass is called the specific energy of that fuel.
In general an engine using that fuel will generate less kinetic energy due to inefficiencies and thermodynamic considerations—hence the specific fuel consumption of an engine will always be greater than its rate of production of the kinetic energy of motion.
The greatest energy source by far is mass itself.
This energy, "E = mc2", where "m = ρV", "ρ" is the mass per unit volume, "V" is the volume of the mass itself and "c" is the speed of light.
This energy, however, can be released only by the processes of nuclear fission (.1%), nuclear fusion (1%), or the annihilation of some or all of the matter in the volume "V" by matter-antimatter collisions (100%).
Nuclear reactions cannot be realized by chemical reactions such as combustion.
Although greater matter densities can be achieved, the density of a neutron star would approximate the most dense system capable of matter-antimatter annihilation possible.
A black hole, although denser than a neutron star, does not have an equivalent anti-particle form.
The highest density sources of energy aside from antimatter are fusion and fission.
Fusion includes energy from the sun which will be available for billions of years (in the form of sunlight) but so far (2011), sustained fusion power production continues to be elusive.
Fission of uranium and thorium in nuclear power plants will be available for a long time due to the vast supply of the element on earth, though the full potential of this source can only be realised through breeder reactors, which are, apart from the BN-600 reactor, not yet used commercially.
Coal, gas, and petroleum are the current primary energy sources in the U.S. but have a much lower energy density.
Burning local biomass fuels supplies household energy needs (cooking fires, oil lamps, etc.)
Energy density (how much energy you can carry) does not tell you about energy conversion efficiency (net output per input) or embodied energy (what the energy output costs to provide, as harvesting, refining, distributing, and dealing with pollution all use energy).
Like any process occurring on a large scale, intensive energy use impacts the world.
For example, climate change, nuclear waste storage, and deforestation may be some of the consequences of supplying our growing energy demands from carbohydrate fuels, nuclear fission, or biomass.
No single energy storage method boasts the best in specific power, specific energy, and energy density.
Peukert's Law describes how the amount of useful energy that can be obtained (for a lead-acid cell) depends on how quickly we pull it out.
To maximize both specific energy and energy density, one can compute the specific energy density of a substance by multiplying the two values together, where the higher the number, the better the substance is at storing energy efficiently.
Gravimetric and volumetric energy density of some fuels and storage technologies (modified from the Gasoline article): Note: Some values may not be precise because of isomers or other irregularities.
See Heating value for a comprehensive table of specific energies of important fuels.
Note: Also it is important to realise that generally the density values for chemical fuels do not include the weight of oxygen required for combustion.
This is typically two oxygen atoms per carbon atom, and one per two hydrogen atoms.
The atomic weight of carbon and oxygen are similar, while hydrogen is much lighter than oxygen.
Figures are presented this way for those fuels where in practice air would only be drawn in locally to the burner.
This explains the apparently lower energy density of materials that already include their own oxidiser (such as gunpowder and TNT), where the mass of the oxidiser in effect adds dead weight, and absorbs some of the energy of combustion to dissociate and liberate oxygen to continue the reaction.
This also explains some apparent anomalies, such as the energy density of a sandwich appearing to be higher than that of a stick of dynamite.