The United States imports a small portion of its electricity from Canada and Mexico, with net imports totaling 0.16 quads. Most of Canada’s electricity comes from hydroelectric power plants while the electricity from Mexico comes largely from the combustion of fossil fuels, mainly natural gas.

Electricity is called secondary energy because it’s derived from other energy sources, mainly coal, natural gas, and nuclear reactions. About 39% of the total U.S. energy supply goes to producing electricity.

Residential use accounted for 21% of total U.S. energy consumption in 2015. The top energy uses in the residential sector were: Space Heating (30%), Water Heating (13%), Space Cooling (10%), and Lighting (9%).

Offices, grocery stores, hospital, sports arenas, shopping malls, and other commercial spaces accounted for 18% of the energy used in the United States in 2015. Most of that energy was used for heating and lighting.

Industry accounted for 32% of the energy used by the United States in 2014. Most of that went to a handful of energy-intensive industries: petroleum refining, chemicals, paper, and metal.

Cars, trucks, buses, trains, ships, and planes compose the transportation sector, which is powered almost entirely by oil. The United States uses 28% of its total energy each year to move people and goods from one place to another.

There are many opportunities to improve our nation’s energy efficiency, but it’s impossible to avoid losing some energy as heat when converting energy from one form to another. The principles of physics place upper limits on how efficient a heat engine, power plant, or oil refinery can be.

Useful energy describes the amount of energy that went toward accomplishing the work that needed to be done, whether it was moving a car, lighting a bulb, or driving a turbine to generate power, as well as direct heat for space heating, cooking, manufacturing, etc.

The enormous quantity of energy consumed in the United States in one year is measured in quadrillion (1 million billion) British thermal units—or quads. One quad is approximately the amount of energy used by a city the size of Chicago in one year.

Sunlight is a free and abundant resource, but converting it into cost-effective electricity sometimes requires feed-in tariffs, subsidies, or power-purchase agreements. Although sunlight is intermittent, one factor favoring solar energy systems is that they produce maximum power close to the time of peak electricity demand each day. % of Total U.S. Output—0.23% % of Total U.S. CO2 emissions—0%

Nuclear power plants provide about one-fifth of the electricity generated in the United States. Nuclear plants don’t emit greenhouse gases or pollutants that are harmful to human health and the environment, but they do create radioactive waste that must be stored safely. % of Total U.S. Output—8% % of Total U.S. CO2 emissions—0%

Hydropower is renewable, free of CO2 emissions, and more reliable than solar or wind energy, except during periods of prolonged drought. Damming rivers and streams is disruptive to local ecosystems and the existing hydropower resource base is limited, but future projects may harness energy from waves, tides, and ocean currents. % of Total U.S. Output—2.5% % of Total U.S. CO2 emissions—0%

Wind energy, now deployed in 39 states, has grown considerably in recent years and has a large potential for further expansion. But as an intermittent source, it still awaits an effective way to store its energy output. % of Total U.S. Output—2% % of Total U.S. CO2 emissions—0%

The heat from Earth’s molten interior provides geothermal energy. It’s chiefly used in the western United States, where proximity to high temperature geothermal fields makes it practical to generate electricity. Additionally, a small number of homes and commercial buildings use geothermal energy directly for heating and cooling. % of Total U.S. Output—0.2% % of Total U.S. CO2 emissions—0%

After coal, natural gas is the second largest contributor to the nation’s supply of electricity. Additionally, more than half of American homes also consume natural gas in stoves, furnaces, water heaters, and clothes dryers. Industry uses natural gas in the production of glass, paper, clothing, and other products. It’s also an essential raw material in fertilizer, plastics, paints, and even medicines. % of Total U.S. Output—29% % of Total U.S. CO2 emissions—28%

Nearly all of the coal mined in the United States goes toward electricity generation, and it is the nation’s largest contributor to the electric supply. Coal also provides direct heat to industries that make steel, concrete, paper, and other essential products. It is relatively inexpensive for its energy content, but coal is also a major source of CO2 emissions and pollutants that impact human health and the environment. % of Total U.S. Output—16% % of Total U.S. CO2 emissions—29%

Biomass—which includes organic material such as wood, methane from landfills, and agricultural crops—is burned to generate electricity or provide direct heat. Industry uses the most biomass, mainly as a heat source. Biomass is also used to make biofuels (such as ethanol and biodiesel), which are often mixed with petroleum-based transportation fuels. % of Total U.S. Output—4% % of Total U.S. CO2 emissions—0%

Oil is the predominant fuel source in the transportation sector—gasoline, diesel, and jet fuel all come from crude oil. Industries use oil for space heating, process heating, and as a raw material for common products such as plastics, ink, detergents, and tires. Additionally, many homes rely on heating oil in the winter. % of Total U.S. Output—36% % of Total U.S. CO2 emissions—43%