Energy fuels everything—from the tiniest atomic motion to massive industrial systems. It’s the invisible force behind motion, heat, light, electricity, and life itself. In physics, energy is simply defined as the capacity to do work or bring about change. Because energy exists in many forms and can easily be transformed from one type to another, energy conversion is central to science, engineering, and everyday living.
What is Energy?
Put simply, energy is what drives everything — from movement and heat to electricity and life itself. We encounter it all the time in daily life: flipping on a light switch, cooking dinner, or even just walking across the room. In scientific terms, energy is measured in joules (J) and follows the law of conservation of energy, which states that energy can’t be created or destroyed — only converted from one form to another.
Since heat is a form of energy, understanding temperature changes is essential. Use our Temperature Converter to switch between Celsius, Fahrenheit, Kelvin, and more.
There are many different types of energy, each essential to how things work in various fields:
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Kinetic Energy: The energy of motion — seen in things like moving vehicles, blowing wind, or running water.
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Potential Energy: Stored energy — like a compressed spring, a raised weight, or water held behind a dam.
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Thermal Energy: Also known as heat energy — caused by the movement of molecules, as seen in boiling water or sunlight.
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Electrical Energy: Energy that flows through electric charges — powering everything from smartphones to factories.
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Chemical Energy: Stored in fuels, food, and batteries — released when substances go through chemical changes.
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Nuclear Energy: The powerful energy found in the nucleus of atoms — used in both nuclear reactors and the fusion that powers the sun.
Looking for more tools related to science and physics? Browse our full Conversion Tools section for deeper learning.
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Energy Units and Conversion
Energy can be expressed in different units depending on the context, with the joule (J) serving as the official SI unit. However, other units are often used in specific fields — for instance, calories (cal) are common for measuring heat or food energy, kilowatt-hours (kWh) are used for electricity, and electron volts (eV) are widely applied in atomic and quantum physics. You’ll also find energy measured in British Thermal Units (BTU) in heating and cooling systems, and foot-pounds (ft-lb) when dealing with mechanical work.
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Energy Unit
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Equivalent in Joules (J)
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1 Joule (J)
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1 J
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1 Kilojoule (kJ)
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1,000 J
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1 Calorie (cal)
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4.184 J
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1 Kilocalorie (kcal, Cal)
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4,184 J
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1 Watt-hour (Wh)
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3,600 J
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1 Kilowatt-hour (kWh)
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3,600,000 J
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1 Electronvolt (eV)
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1.602 × 10⁻¹⁹ J
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1 Megajoule (MJ)
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1,000,000 J
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1 Gigajoule (GJ)
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1,000,000,000 J
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1 British Thermal Unit (BTU)
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1,055.06 J
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1 Therm
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105,505,585 J
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1 Foot-pound (ft-lb)
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1.3558 J
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1 Horsepower-hour (hp.h)
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2,685,000 J
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1 Hartree (Ha)
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4.3597 × 10⁻¹⁸ J
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1 Planck Energy
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1.956 × 10⁹ J
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1 Ton of TNT (tTNT)
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4,184,000,000 J
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1 Erg
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1 × 10⁻⁷ J
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1 Thermochemical Calorie (cal_th)
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4.184 J
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Because different industries rely on different units, energy conversions are often required. For example, your utility bill shows electricity usage in kilowatt-hours, while food packaging displays calories. To maintain precision in scientific research, engineering, or daily applications, converting between energy units is not just helpful — it’s essential. The following table outlines standard conversion values between the most frequently used energy units.
🍽️ Want to know how energy applies to food and the human body? Try our Calorie Calculator to see how much energy your meals provide.
How to Calculate Energy?
Calculating energy depends on the specific type you're working with — whether it's kinetic, potential, thermal, or electrical energy. One of the most widely used equations in physics is the formula for kinetic energy, which is expressed as:
Kinetic Energy
Energy calculations vary depending on the form of energy being measured — whether it’s kinetic, potential, or electrical. One of the most familiar formulas in physics is for kinetic energy, written as:
KE = ½ mv²
Where KE represents kinetic energy in joules (J), m is mass in kilograms (kg), and v is velocity in meters per second (m/s).
For example, if an object weighs 10 kg and moves at 5 m/s, the calculation would be:
KE = ½ × 10 × 5² = 125 J
You can calculate kinetic energy instantly with our Kinetic Energy Calculator — just enter mass and velocity.
Potential Energy
PE = mgh
Here, PE is potential energy, m is mass, g is the acceleration due to gravity (9.81 m/s²), and h is height in meters.
If a 20 kg object is raised to a height of 3 meters, the potential energy would be:
PE = 20 × 9.81 × 3 = 588.6 J
If you're working with heights and gravity, our Potential Energy Calculator makes it easy to find stored energy values.
Electrical energy
To determine electrical energy, the equation is:
E = P × t
Where E is energy in joules (J), P is power in watts (W), and t is time in seconds.
So, if a 100 W lightbulb is on for 2 hours (7200 seconds), it uses:
E = 100 × 7200 = 720,000 J, or 720 kJ.
The Largest Energy Consumption in History
In 2022, the world consumed over 600 exajoules of energy — that’s 600 billion billion joules. To put it into perspective, it’s the equivalent of detonating more than 143 million tons of TNT. Massive numbers like these are why understanding energy conversion is critical. Whether discussing climate change, national power grids, or renewable energy, converting between joules, kilowatt-hours, or BTUs ensures that global energy use is measurable, comparable, and manageable.
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How Much Energy is in a Cheeseburger?
Ever wondered how much energy you actually eat? A typical fast-food cheeseburger contains about 300 kilocalories, which is roughly 1.26 megajoules. That’s enough energy to keep a 100-watt light bulb glowing for about 3.5 hours. It’s a fun reminder that our bodies are energy machines too — and that converting food calories into mechanical or electrical equivalents is a clever way to see physics in everyday life. Energy conversion isn't just for engineers — it’s in your lunchbox too.
