Updated: May 25, 2023
By Drew Schiavone
Electrical Outlet

Energy is defined as the ability to do work. Modern civilization is possible because people have learned how to change energy from one form to another and then use it to do work. There are many different forms of energy, including heat, light, motion, electrical, chemical and gravitational. One practical example of energy use within our society, includes the conversion of the stored chemical energy in coal or natural gas into electrical energy. This electrical energy can then be converted into the light or heat energy used in our homes, farms and businesses.

What is energy?

How is energy defined?

Energy is the ability to do work. There are many different forms of energy, all of which  can be grouped into one of two general types of energy; potential (stored energy) and kinetic (working energy). Energy can be converted from one form to another. For example, the stored chemical energy in coal or natural gas, and the kinetic energy of water flowing in rivers, can be converted to electrical energy, which in turn can be converted to light and heat. 

POTENTIAL ENERGY (stored energy) is the energy of position  EXAMPLES
Chemical energy is energy stored in the bonds of atoms and molecules. Chemical energy can often be converted to thermal energy (e.g., burning wood in a fireplace, burning gasoline in a engine. batteries, biomass, coal, natural gas, petroleum
Gravitational energy is stored in an object’s height. More gravitational energy will be stored in higher and heavier objects. Gravitational energy can often be converted to motion energy (e.g., biking down a hill.) hydropower
Nuclear energy is energy stored in the nucleus of an atom—the energy that holds the nucleus together. Large amounts of energy can be released when the nuclei are combined or split apart. nuclear


KINETIC ENERGY (working energy) is the energy of motion EXAMPLES
Electrical energy is delivered by tiny charge particles called electrons, typically moving through a wire. Lightning is an example of electrical energy in nature. electricity
Motion energy is energy stored in the movement of objects. The faster they move, the more energy is stored. It takes energy to get an object moving, and energy is released when an object slows down. wind
Thermal energy (heat) is the energy that comes from the movement of atoms and molecules in a substance. Heat increases when these particles move faster. combustion, geothermal, solar (active and passive)


Are power and energy different?

The terms "power" and "energy" are often used interchangeably in conversation, but they have different meanings. Energy (often measured in Watt-hour) is the capability to do work, while power (often measured in Watts) is defined as energy consumed per unit time. Therefore, power refers to the rate of energy being used, generated, or transferred. 



Are power and energy different?

The terms "power" and "energy" are often used interchangeably in conversation, but they have different meanings. Energy (often measured in Watt-hour) is the capability to do work, while power (often measured in Watts) is defined as energy consumed per unit time. Therefore, power refers to the rate of energy being used, generated, or transferred.

Lightbulb Diagram for Power and Energy

What is a primary energy source?

Primary energy sources (i.e., petroleum, coal, natural gas, nuclear fuel) are those that are used to make secondary sources of energy (i.e., electricity). When people use electricity at their home, farm or business, the electrical power is probably generated by burning coal or natural gas, by a nuclear reaction, or by a hydroelectric plant on a river, to name just a few sources. When people fill up the gas tank on their car, the energy source is petroleum (gasoline) refined from crude oil and may include fuel ethanol made by growing and processing corn. The top three primary energy sources consumed in Maryland are petroleum (33%), natural gas (23%), and nuclear electric power (12%).

Maryland Energy Consumption Estimates 2018 Graph

What is electrical energy?

What is electricity?

Electricity (or electric current) is the flow of electric power or electric charges from one point to another. Electricity is measured in Amperes. The electricity that we use for every day applications (lighting, heating, cooling) is a secondary energy source since it is produced from primary energy sources such as coal, natural gas, nuclear energy, solar energy and wind energy. Electricity is also referred to as an energy carrier, which means it can be converted to other forms of energy such as mechanical energy or heat. 

Electron Flow and Current Illustration

How is electricity generated in Maryland?

Nuclear and natural gas-fired power plants supply about 66% of Maryland's electricity net generation. Maryland's only nuclear power plant (Calvert Cliffs) accounted for 34% of the state's electricity net generation in 2018. Natural gas-fired generation has more than tripled since 2015 and accounted for 32% of the state's net generation in 2018. Coal-fired generating plants historically supplied more than half the state's net electricity generation, but coal's share has been below 50% since 2012 and fell to 23% in 2018 as natural gas-fired generation increased. As of mid-2019, all but two of the 14 generating units at Maryland's seven remaining coal-fired power plants were more than 30 years old. Three of those older units were scheduled to shut down in 2019 and in 2020. Hydroelectric power and other renewable energy sources account for most of the state's remaining utility-scale net generation. Maryland encourages construction of generating facilities to meet growing electricity demand, and since 2015 almost all the state's new generating capacity has been natural gas-fired or solar powered.

Maryland Net Electricity Generation by Source November 2020 Graph

How is electricity used in Maryland?

Demand for electricity has grown over the long-term, however, the growth has slowed progressively each decade since the 1950’s. From 2000 to 2009, increases in electricity demand averaged 0.5 percent per year, with demand growth projected to be approximately 1% per year through 2035. (EIA DOE, 2011). While roughly 39% of total United States electricity consumption in 2009 was residential, more than 90% of the electricity consumed in Maryland is currently used by the commercial and the residential sectors combined. The state also pursues efficiency goals to reduce the use of electricity. The state also pursues efficiency goals to reduce electricity use. The state's public utility commission set an energy efficiency goal for retail electricity distributors—a 2% reduction in retail electricity sales by 2020.

Maryland Energy Consumption by End Use 2018 Graph US Residential Electricity Consumption 2018 Graph

Electricity in the home is primarily used to power lights and appliances (56%), condition air (22%), and heat water (9%). Four in 10 Maryland households use electricity as their primary heating source. In agriculture, electricity is used for irrigation pumps, indoor and outdoor lighting, grain drying, powering farm shops, and livestock water systems. Average consumption in agriculture is difficult to determine because of the many and varied types of agricultural operations.

What is thermal energy?

What is heat?

Thermal energy (also called heat energy) is produced when a rise in temperature causes atoms and molecules to move faster and collide with each other. The energy that comes from the temperature of the heated substance is called thermal energy. The molecules and atoms that make up matter are moving all the time. When a substance heats up, the rise in temperature makes these particles move faster and bump into each other. Thermal energy is the energy that comes from the heated up substance. The hotter the substance, the more its particles move, and the higher its thermal energy.

Heat Transfer Diagram of Boiling Pot over Burning Firewood

How is thermal energy generated in Maryland?

Maryland has few economically recoverable natural gas reserves, and the state produces very little natural gas. The few low-production wells in western Maryland collectively produce less than 40 million cubic feet of natural gas annually. While Maryland's westernmost counties overlie part of the natural gas-rich Marcellus Shale, the state enacted a permanent ban on using hydraulic fracturing in 2017. Hence, the state's natural gas needs are met by supplies that enter the state by interstate pipelines and a liquefied natural gas (LNG) port. Maryland also has no economically recoverable crude oil reserves or production, and there are no petroleum refineries in the state. Petroleum products arrive in Maryland by pipelines delivering refined products (e.g., motor gasoline, kerosene, home heating oil, diesel fuel) and by ships carrying imported petroleum products (e.g., motor gasoline, gasoline blendstocks, residual fuel oil). Propane is a petrochemical that is either removed from natural gas before entering a pipeline or a byproduct of oil refining (e.g., heating oil, petroleum). Currently, natural gas and oil refining produce enough propane to meet around 90% of U.S. annual fuel needs.

Calculated combustion energy use by fuel type in 2014 chart
Calculated combustion energy use by fuel type in 2014 for target industries. McMillan, C., Boardman, R., McKellar, M., Sabharwall, P., Ruth, M., & Bragg-Sitton, S. (2016). Generation and use of thermal energy in the US Industrial sector and opportunities to reduce its carbon emissions (No. NREL/TP-6A50-66763; INL/EXT-16-39680). National Renewable Energy Lab.(NREL), Golden, CO (United States).

Maryland also holds about 0.1% of the nation's estimated recoverable coal reserves and accounts for about 0.2% of U.S. coal production. The state has fewer than 15 surface and underground coal mines, all of them located in the Appalachian Basin in the state's western counties. About 75% of the coal mined in Maryland is sent to electric power generators, with the remainder burned at industrial facilities. Overall, coal mined in Maryland provides less than one-fifth of the coal that is consumed by the state's coal-fired power plants. There may be as much as 780,000 dry tons of wood biomass available annually distributed across the state. Wood chips from forest management (e.g. thinnings and final harvests) are available in rural communities with logging capacity, particularly in the western region and on the Delmarva Peninsula. Urban wood waste, Maryland’s largest potential source of wood biomass, is available statewide, with the greatest concentration occurring in the central region where over half of the state’s 35 Natural Wood Waste Recycling (NWWR) facilities operate. Wood pellets are available at retail outlets throughout Maryland but no pellet manufacturers currently exist in state. The cost of production notwithstanding, more than 600,000 green tons of wood could be available from short rotation wood energy crops cultivated on Maryland’s idle lands annually. Maryland’s firewood market is significant, with more than 260 firewood contractors and many more landowners harvesting cordwood for local consumption.

How is thermal energy used in Maryland?

Around 40% of all energy consumed in Maryland is thermal energy in the form of heating and cooling for buildings and industrial processes. Maryland overwhelmingly relies on fossil energy—propane (3%), natural gas (47%), number 2 heating oil (11%), and electricity (39%) mostly from coal and nuclear—for heating and cooling. Biomass (cord wood and pellets, and wood chips in some commercial applications) makes up a relatively small portion of the thermal energy use in Maryland. Maryland's residential and commercial sectors each account for about one-third of the state's natural gas consumption, and the electric power sector uses nearly one fourth. Maryland's per capita petroleum consumption is among the lowest in the nation. While the transportation sector accounts for the vast majority (90%) of the state's petroleum consumption, the industrial sector uses about 6% and the residential and commercial sectors consume about 4%. More than 4 out of 10 Maryland households use natural gas as their primary fuel for home heating. About 1 in 8 Maryland households use fuel oil or kerosene for heating.An estimated 18% of Maryland homes have at least one wood combustion appliance, consuming an estimated 340,000 tons of wood in aggregate each year.

Biomass Combustion in Maryland Homes pie chart
Kays, J. S. (2012). A Prospectus For Advancing Biomass Thermal Energy In Maryland Developed By the Maryland Wood Energy Coalition.

Further Information

Calculating Energy Content

Kilowatts are only one way of measuring energy. We also measure energy in terms of tons, cubic feet, gallons, and barrels. To compare fuels with each other, they must be converted to the same units, such as British Thermal Units (commonly referred to as "Btu"). A Btu is a precise measure of the heat content of fuels. The Btu content of each fuel provided below (except for crude oil) is the average heat content for fuels consumed in the United States (preliminary estimates for 2018):

BTU Content of Common Energy Sources
Source Unit BTU Potential
Crude Oil 1 Barrel (42 gallons) 5,705,000
Gasoline 1 Gallon 120,333
Heating Oil 1 Gallon 137,381
Diesel Fuel138,500 1 Gallon 138,500
Residual Fuel Oil 1 Barrel (42 gallons) 6,287,000
Natural Gas 1 Cubic Foot 1,036
Propane 1 Gallon 91,333
Coal 1 Short Ton 18,911,000
Electricity 1 Kilowatt-Hour (kWh) 3,412


Example: You have a natural gas furnace in your home that used 67,000 cubic feet of natural gas for heating last winter. Your neighbor has a furnace that burns heating oil that used 500 gallons of heating oil last winter. You can convert the natural gas and heating oil consumption data into Btu to determine which home used more energy for heating.
  Current Use Units   BTU Content   Current Use BTU
Natural Gas
(Your Home)
67,000 cubic feet x 1,036 BTU
per cubic feet
= 69,412,000 BTU
Heating Oil
500 gallons x 137,381 BTU
per gallon
= 68,690,476 BTU
Result: You used more energy to heat your home. (Note that many factors affect the amount of energy a household actually uses for heating.)


Example: You and your neighbor want to compare the price of the fuels for heating your homes on an equal basis. You can compare the fuel prices in dollars per million Btu by dividing the price per unit of the fuels by the Btu content of the fuels in million Btu per unit.
  Cost per Unit   BTU Content   Cost per BTU
Natural Gas $12.60
per thousand cubic feet
÷ 1.036 BTU
per thousand cubic feet
= $12.16
per million BTU
Heating Oil $3.54
per thousand cubic feet
÷ 0.137381 million BTU
per gallon
= $25.76
per million BTU
Result: The price per million Btu for natural gas is about half the price of heating oil per million Btu.


For more energy conversions, visit the EIA website (Energy Conversion Calculatorwhich offers several energy conversion calculators


Additional Resources

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