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.

With less than 5% of the world's population, the United States is home to one-third of the world's automobiles.

With less than 5% of the world's population, the United States is home to one-third of the world's automobiles.

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

A few industries use a very large share of energy in the industrial sector. Petroleum refining is the principal consumer, with the chemical industry a close second. Those users, plus the paper and metal industries, account for 78% of total industrial energy use.

A few industries use a very large share of energy in the industrial sector. Petroleum refining is the principal consumer, with the chemical industry a close second. Those users, plus the paper and metal industries, account for 78% of total industrial energy use.

A few industries use a very large share of energy in the industrial sector. Petroleum refining is the principal consumer, with the chemical industry a close second. Those users, plus the paper and metal industries, account for 78% of total industrial energy use.

A few industries use a very large share of energy in the industrial sector. Petroleum refining is the principal consumer, with the chemical industry a close second. Those users, plus the paper and metal industries, account for 78% of total industrial energy use.

School buildings represent 13% of commercial buildings energy use, or about 2.5% of total U.S. energy use (13% × 19%).

School buildings represent 13% of commercial buildings energy use, or about 2.5% of total U.S. energy use (13% × 19%).

School buildings represent 13% of commercial buildings energy use, or about 2.5% of total U.S. energy use (13% × 19%).

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. 

As of 2014, the very best experimental units could convert more than 40% of light energy to electricity; ordinary commercial units are in the range of 5% to 20%. 

As of 2014, the very best experimental units could convert more than 40% of light energy to electricity; ordinary commercial units are in the range of 5% to 20%. 

As of 2014, the very best experimental units could convert more than 40% of light energy to electricity; ordinary commercial units are in the range of 5% to 20%. 

CO2 concentration in the atmosphere has risen about 43% since the beginning of the industrial revolution in the mid-eighteenth century-half of that since 1980

CO2 concentration in the atmosphere has risen about 43% since the beginning of the industrial revolution in the mid-eighteenth century-half of that since 1980

CO2 concentration in the atmosphere has risen about 43% since the beginning of the industrial revolution in the mid-eighteenth century-half of that since 1980

Carbon capture and storage will reduce energy efficiency of a coal plant, though it will decrease carbon emissions.