Fuel cells are an efficient way to convert hydrogen to electricity, but the energy is stored in the hydrogen.

For decades, more than half of all residential energy use went  to space heating and cooling; in 1993, it accounted for nearly 60%. But EIA data show that by 2009, that share had dropped to 48%. And in the period 1993 to 2009, energy for appliances, electronics, and lighting rose from 24% to 35%, owing to the proliferation of appliances, as well as trends toward larger TVs and other devices.

Electricity is a secondary energy source because it can only be produced from the use of primary energy sources such as coal, natural gas, or nuclear reactions.

Electricity is a secondary energy source because it can only be produced from the use of primary energy sources such as coal, natural gas, or nuclear reactions.

Electricity is a secondary energy source because it can only be produced from the use of primary energy sources such as coal, natural gas, or nuclear reactions.

Electricity is a secondary energy source because it can only be produced from the use of primary energy sources such as coal, natural gas, or nuclear reactions.

U.S. energy consumption is projected to rise 9% by 2040, or 0.3% per year, while global consumption will increase about 50% over the same period

U.S. energy consumption is projected to rise 9% by 2040, or 0.3% per year, while global consumption will increase about 50% over the same period

U.S. energy consumption is projected to rise 9% by 2040, or 0.3% per year, while global consumption will increase about 50% over the same period

A typical incandescent lamp (traditional light bulb) that consumes 60 watts of power produces around 800 lumens. A compact fluorescent lamp emits the same amount of light while using only 13 watts. And an LED lamp consumes only 10 watts to give off the same 800 lumens.

A typical incandescent lamp (traditional light bulb) that consumes 60 watts of power produces around 800 lumens. A compact fluorescent lamp emits the same amount of light while using only 13 watts. And an LED lamp consumes only 10 watts to give off the same 800 lumens.

A typical incandescent lamp (traditional light bulb) that consumes 60 watts of power produces around 800 lumens. A compact fluorescent lamp emits the same amount of light while using only 13 watts. And an LED lamp consumes only 10 watts to give off the same 800 lumens.

In 2014, the United States got 35% of its energy from petroleum, and experts project that demand for this fuel will rise at least through 2020. 

In 2014, the United States got 35% of its energy from petroleum, and experts project that demand for this fuel will rise at least through 2020. 

In 2014, the United States got 35% of its energy from petroleum, and experts project that demand for this fuel will rise at least through 2020. 

The British Thermal Unit, or Btu, is frequently used as a measure for energy content of fuels. One gallon of gasoline contains about 124,000 Btu.

The British Thermal Unit, or Btu, is frequently used as a measure for energy content of fuels. One gallon of gasoline contains about 124,000 Btu.

The British Thermal Unit, or Btu, is frequently used as a measure for energy content of fuels. One gallon of gasoline contains about 124,000 Btu.

Tidal energy is gravitational in origin. Solar energy comes from nuclear reactions in the sun.

Tidal energy is gravitational in origin. Geothermal energy comes from radioactive decay inside the earth.

Tidal energy is gravitational in origin. Solar energy comes from nuclear reactions in the sun, and geothermal energy comes from radioactive decay inside the earth.

By far the largest share of energy in transportation is consumed by cars, light trucks, and motorcycles—about 58% in 2012, followed by other trucks (21%), aircraft (9%), boats and ships (3%), and trains and buses (3%). Pipelines account for 3% and military uses for 2%.

By far the largest share of energy in transportation is consumed by cars, light trucks, and motorcycles—about 58% in 2012, followed by other trucks (21%), aircraft (9%), boats and ships (3%), and trains and buses (3%). Pipelines account for 3% and military uses for 2%.

By far the largest share of energy in transportation is consumed by cars, light trucks, and motorcycles—about 58% in 2012, followed by other trucks (21%), aircraft (9%), boats and ships (3%), and trains and buses (3%). Pipelines account for 3% and military uses for 2%.

By far the largest share of energy in transportation is consumed by cars, light trucks, and motorcycles—about 58% in 2012, followed by other trucks (21%), aircraft (9%), boats and ships (3%), and trains and buses (3%). Pipelines account for 3% and military uses for 2%.