Is Energy A Watt Or Joule?
Energy is a fundamental concept in physics and is critical to understanding how our world works. From the food we eat to power our homes, energy is deeply woven into every aspect of our lives. Given how ubiquitous energy is, it’s important to have a clear grasp of how it is measured and quantified.
Understanding the units used for energy allows us to make useful comparisons and calculations in daytoday life. For example, being able to convert between units like calories, kilowatthours, and joules enables us to evaluate the energy content of food, estimate our electricity usage, and generally make sense of the energy concepts we encounter. The proper use of key terms like power and energy also helps avoid confusion when learning about related physics principles.
In this article, we will demystify the units used to measure energy. We’ll examine the differences between energy and power units, explain what a watt and joule represent, provide examples of their usage, and outline how to convert between energy units. Developing an intuition for energy units is a valuable skill for both science students and nonexperts interested in gaining a better understanding of the role energy plays in our lives.
Defining Energy
Energy is the ability to do work or produce heat. It is a quantitative property of a system which may be kinetic, potential, or internal. Kinetic energy is the energy of motion. Potential energy is stored energy based on an object’s position or arrangement. Internal energy is the total of all microscopic forms of energy of a system. Energy is measured in joules (J) in the International System of Units (SI).
Energy can be transferred between objects or converted between different forms, but it cannot be created or destroyed according to the law of conservation of energy. For example, a swinging pendulum converts potential energy to kinetic energy and back again over time. Energy flows or transfers due to differences in temperature, pressure, or chemical potential. Life, technology, and all activity require energy transfers and transformations.
Units of Energy
Energy is measured in a variety of units depending on the field and application. However, the standard SI unit for energy, used across science and engineering, is the joule.
The joule, symbolized as J, is defined as the amount of work done by a force of one newton exerted over a distance of one meter. One joule is equal to the energy exerted to accelerate one kg of mass at 1 m/s2 over a distance of 1 meter.
Some other common units of energy include:
 Calorie (cal) – often used in nutrition and defined as the energy needed to raise 1 gram of water by 1°C.
 Kilowatt hour (kWh) – commonly used for electrical energy and equal to 3,600,000 joules.
 British thermal unit (BTU) – used for heat energy, with 1 BTU equal to about 1055 joules.
 Barrel of oil equivalent (BOE) – used for energy from fossil fuels, with 1 BOE approximately equal to 6,000,000,000 joules.
While these other units are in use, the joule represents the standard base unit for all energy measurements in the International System of Units.
Power vs Energy
Power and energy are related but distinct concepts. Power refers to the rate at which energy is transferred or converted per unit of time. Energy, on the other hand, is the capacity to do work. Energy comes in many forms like chemical, mechanical, thermal, electrical, etc.
Power is measured in watts (W), which equals one joule (J) of energy transferred per second. For example, a 100watt lightbulb uses 100 joules of electrical energy every second. Energy is measured in joules (J). A joule is a derived unit of energy defined as the work done by a force of one newton acting over a distance of one meter.
To summarize, power describes the rate of energy flow, whereas energy is the total amount of capacity to do work. Power is measured in watts, and energy is measured in joules. Although related, power and energy are distinct physical concepts.
Watt: Unit of Power
The watt is a unit of power equal to 1 joule per second. Power describes the rate at which energy is transferred, generated, or used over time. While the joule quantifies energy, the watt quantifies power.
For example, a 100watt light bulb uses 100 joules of energy every second. If left on for an hour, it would use 100 joules x 3,600 seconds = 360,000 joules of energy. The watt measures how quickly the light bulb uses energy, while the joule measures the total amount of energy used.
Some key facts about the watt:
 Defined as 1 joule per second (J/s).
 Measures the rate of energy transfer or usage.
 Used to quantify electrical power, mechanical power, and radiant flux.
 Commonly used in describing light bulbs, motors, appliances, and solar panels.
So in summary, the watt is a unit of power, while the joule is a unit of energy. Watts measure the rate, joules measure the total amount.
Using Energy Units
When discussing energy, it’s important to understand the difference between units that measure power (energy transfer rate) versus units that measure total energy. The joule is the unit of energy, while the watt is the unit of power.
Joules (abbreviated as J) measure the total amount of energy something contains or the total amount of work done. For example, a battery stores a certain number of joules of chemical energy. When you expend energy doing work, like lifting a heavy box, you use up a certain number of joules. The total energy is measured in joules.
Watts (abbreviated as W), on the other hand, measure the rate of energy transfer or power. Power describes how quickly or slowly energy is being used or transferred. For example, a lightbulb may use 100 watts of power, meaning it converts 100 joules of electrical energy into light and heat energy every second. The rate of energy transfer is measured in watts.
To summarize, joules measure total energy while watts measure the rate that energy is produced, used, or transferred. Knowing the difference between these units is essential when learning about energy.
Energy Conversion
Energy can be expressed in different units depending on the context. While the joule is the standard unit of energy, the watt is a unit of power. Power and energy are related but distinct concepts.
Power describes the rate at which energy is transferred or converted per unit of time. It is measured in watts (W). Energy is the capacity to do work. It is measured in joules (J).
Watts and joules are connected through time. The same amount of power applied over different lengths of time will result in different amounts of energy being transferred. Specifically, watts can be converted to joules by multiplying by time in seconds:
1 Watt x 1 Second = 1 Joule
So a 100W lightbulb uses 100 joules of energy per second. Over the course of an hour, it would use 100 x 60 x 60 = 360,000 joules. This demonstrates how watts can quantify the rate of energy usage, while joules measure the total energy.
Everyday Examples
Energy is a concept we encounter frequently in everyday life. Here are some common examples that demonstrate energy units:

Lightbulbs are rated based on their wattage, which refers to the rate at which they consume energy. A 60W lightbulb uses 60 joules of energy per second when turned on.

Batteries store energy in units of watthours (Wh), which is power multiplied by time. A battery rated for 100 Wh can deliver 100 watts of power for 1 hour before being depleted.

Gasoline is sold by the gallon in the US, but gasoline pumps also show the energy content in megajoules (MJ). A gallon of gasoline contains about 121 MJ of energy.

Appliances like refrigerators and air conditioners have energy efficiency ratings listed in kWh/year. This denotes the kilowatthours of energy they consume over a year of typical use.

Utility bills for electricity usage are calculated in kWh based on the electric power draw over time. The more energy used, the higher the bill.
These examples illustrate how we routinely quantify, purchase, and consume energy in forms like electricity, fuel, and batteries. The units of watts and joules help standardize energy measurement across different sources and uses in our lives.
Summary
To recap, joules and watts are both units used to measure energy, but in different ways. Joules measure the total amount of energy in a system. For example, a battery stores a certain number of joules. Watts measure the rate of energy transfer or power. For instance, a lightbulb consumes energy at a rate of 60 watts. While joules quantify energy, watts quantify power, which is the rate of energy consumption per unit time. The joule measures the total work done, while the watt measures the rate at which that work is done. In summary, joules are a unit of total energy, while watts are a unit of power.
Conclusions
By now we have explored the fundamental difference between power and energy and examined how watts and joules are used as units of measurement. Understanding the difference between power and energy allows us to properly measure and quantify these distinct physical concepts.
Power describes the rate at which energy is converted or transmitted. It measures how quickly or slowly energy is being used or generated. Watts quantify power. By contrast, energy refers to the capacity to do work. It exists in various forms and can be stored or converted between them. Joules quantify energy.
Knowing when to use watts, when to use joules, and how to convert between them enables accurate calculations and measurements. Whether dealing with electricity, motion, heat, light or any other forms of energy, applying the right units aids understanding and supports useful applications across science, engineering, and daily life.
While watts and joules involve different quantities, they are fundamentally related. The ability to move between power and energy units expands our insight into physical systems and processes. Distinguishing power from energy provides a clearer picture of how energy flows and changes in our universe.