How Do We Define and Measure Energy?

What is energy and why is it important? 

By definition, energy is the capacity to do work. Energy, derived from various natural resources, can be converted into heat that generates power to heat our homes and turn the lights on. Without energy our quality of life would not be how we know it today. Energy consumption is dramatically increasing around the world everyday as more people desire that same lifestyle quality. Think about the common household appliances such as a refrigerator or television. Also, think about your daily commute to school, whether in a car or school bus. Without energy, none of that would be possible! 

Measuring Energy

We use several different units of energy measurement for energy supply and consumption such as:

  • British Thermal Unit (BTU)
  • Quad
  • Barrel of Oil Equivalent (BOE)
  • Joule (J)
  • kilowatt-hour (kWh) 

British Thermal Unit (BTU) and oilfield units 

Natural resources such as oil and gas are a chemical energy source. In addition to their portability, these resources are valued for their dense energy or heat content, primarily measured in British Thermal Units (BTU’s), often referred to as oilfield units. A single (1) BTU will heat 1 pound of water by 1℉. In oilfield units, we commonly use MBTU to represent 1,000 BTU and MMBTU to represent 1,000,000 BTU. 


When referring to measurements of “energy consumption” we often use “quads.” For example, the United States consumes 90 quads of energy annually and one quad of energy is equivalent to 1 quadrillion BTU (1015). 

Barrel of Oil Equivalent (BOE)

Another unit of measurement is barrel of oil equivalent (BOE). This unit allows comparison of various primary energy sources by relating them to oil, which is measured in barrels (=42 gallons). For example, the energy produced or consumed by a country from coal may be reported in BOE, allowing an easy comparison to oil production and consumption in terms of energy content. The BOE unit is frequently used to report how much oil or gas (“reserves”) can be extracted from a field during the exploration and production phase. A single BOE is equivalent to 5.8 * 106 BTU. 

1 Barrel of Crude Oil = 42 gallons = 5.8 million BTU’s 

Joule (J) and the metric/SI system 

In the International System of Units (SI), otherwise known as the metric system, we measure energy in joules. By definition, “one joule equals the work done (or energy expended) by a force of one newton (N) acting over a distance of one meter (m).1Wikimedia Foundation. (2022, January 22). Joule (unit). Wikipedia. Retrieved January 23, 2022 from 

Therefore, 1 Joule (J) equals 1 N * m. Here are some handy conversions and approximations to the SI system from other units previously presented:

1 BTU  equals 1055 J. 

Therefore, 1 quad equals 1.055 ExaJoules (1*1018 J). In the spirit of ‘rule-of-thumb’ engineering analysis, we can recognize that 1 quad is a close approximation to 1 exajoule (EJ) and we can talk about the two as equivalent when we are not worried about precision, for example when talking about energy consumption of countries.

kilowatt-hour (kWh) and the power industry 

Energy can be converted into heat to generate power. When measuring energy consumption for power we use kilowatt-hours (kWh) as the unit of measurement: 1 kWh equals 1 kilowatt of power delivered on a time basis of1 hour. We can convert kWh into joules and BTU’s. To convert kWh into J we use several conversion factors to change the unit of kWh to an equivalent quantity expressed with J. This mathematical approach is termed dimensional analysis and is shown in the equation below:

\[ 1\  \text{kWh} \times \frac{1000\  \text{Wh} }{\text{kWh} } \times \frac{1\frac{\text{J} }{\text{s} } }{\text{W} } \times \frac{3600\  \text{s} }{\text{h} } =3{,}600{,}000\  \text{J}  \]

Therefore 1 kwh equals 3,600,000 J. In a second step, we can then convert 3,600,000 J into BTU to determine the equivalent BTU per kwh. The conversion factor needed is that a single BTU is equivalent to 1055 J. Using dimensional analysis, we solve the following equation: 

\[3{,} 600{,} 000\ \text{J} \times \frac{1\ \text{Btu} }{1055\ \text{J} } =3{,} 412\ \text{Btu} \]

Thus, we now have some additional conversion factors we can apply to other energy conversion problems we may need to solve in the future:

\[\begin{array}{c}1\ \text{kWh} =3{,} 600{,} 000\ \text{J} \\ \\ 1\ \text{kWh} =3{,} 412\ \text{Btu} \end{array} \]

The cost of energy 

Now that we understand measurements of energy, how much does a BTU of energy cost? On average, a single kwh of energy, equivalent to 3,400 BTU, costs 10 cents ($0.10). In regard to BOE, one barrel (bbl) of oil costs roughly $75.00. 

What if you had one dollar, how many BTU’s of energy could you purchase? 

Natural gas = 270,000 BTU/ $1.00 

Oil = 77,000 BTU/ $1.00 

Gasoline = 40,000 BTU/ $1.00 

Electricity = 34,000 BTU/ $1.00

Power and energy are not the same thing. Power is the rate at which energy is used or work is done. For example, we burn fossil fuels, such as natural gas to generate steam (an example kinetic energy source shown below) that is responsible for turning a turbine and as a result, powering an electricity generator.2U.S. Energy Information Administration (EIA) independent statistics and analysis. How electricity is generated. U.S. Energy Information Administration (EIA). (n.d.). The electricity from the generator is then delivered to consumer’s homes through high-voltage transmission lines. 

Based on the table above, which energy source is the most expensive? 
Natural Gas








In the next topic page, let’s consider why that energy source is the most expensive on the list. 

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