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Energy Conversion

Energy is one of the most fundamental concepts present in every physical process – from nutrition to the operation of household appliances. Although we use and consume energy every day, we often don't think about what exactly the numbers mean that we see on food labels, electricity bills, or in physical formulas. In this guide, we present the most important energy units – joule, calorie, kilowatt-hour, and BTU – as well as their conversion, history, and practical applications.

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The Concept and Origin of Energy

The word energy comes from the Greek term energeia, meaning "activity" or "operation." According to the scientific definition, energy is a system's ability to perform work. The history of units of measurement is closely intertwined with the industrial revolution and the development of physics.

In the 19th century, the experiments of English physicist James Prescott Joule proved the law of conservation of energy: energy is not lost, it is only transformed. The joule (J) named after him is today the base unit of the SI (International System of Units) for measuring energy. Later, with the development of thermodynamics, the calorie became important, especially in nutrition and heat measurement. In modern times, the unit of electrical consumption, the kilowatt-hour (kWh), and the BTU (British Thermal Unit) used in Anglo-Saxon areas became widespread.

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Main Energy Units and Their Definitions

Joule (J)

The joule is the SI unit of energy and means the work done by a force of one newton over one meter:

1 J = 1 N × 1 m = 1 kg·m²/s²

This base unit is applicable in every physical process, whether it's kinetic energy, heat, or electrical energy.

Calorie (cal)

The calorie originally served to measure heat energy. One calorie is the amount of energy that raises the temperature of 1 gram of water by 1 degree Celsius. In nutrition, however, we use the kilocalorie (kcal), which equals 1000 calories.

• 1 cal = 4.184 J
• 1 kcal = 4184 J

The "calorie" visible on food labels actually means kilocalorie, so if a chocolate bar contains 250 "calories," it equals 1,046,000 joules of energy.

Kilowatt-hour (kWh)

The kilowatt-hour is the practical unit of electrical energy that shows how much work a 1-kilowatt device performs in 1 hour.

1 kWh = 3.6 × 10⁶ J

This unit is used to measure household energy consumption. An average refrigerator, for example, consumes about 150–200 kWh of energy annually.

BTU (British Thermal Unit)

The BTU is a unit widespread in the Anglo-Saxon world, especially in heating and cooling systems. One BTU is the amount of energy that raises the temperature of 1 pound of water by 1 degree Fahrenheit.

1 BTU = 1055.06 J

The power of air conditioning systems, for example, is often given in BTU/h (BTU per hour).

Conversion Formulas

The following table helps navigate between energy units:

Unit	Abbreviation	Expressed in Joules
Joule	J	1
Calorie	cal	4.184
Kilocalorie	kcal	4184
Kilowatt-hour	kWh	3,600,000
BTU	—	1055.06

Some common conversions:

• 1 kWh = 860.42 kcal = 3.412 × 10³ BTU
• 1 kcal = 1.163 × 10⁻³ kWh
• 1 BTU = 0.252 kcal = 0.000293 kWh

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Practical Examples of Using Energy Units

Nutrition and Human Body

The human body needs continuous energy input for its functioning. The energy consumed during meals is typically displayed in kilocalories. For example, the daily energy requirement of an average adult is 2000–2500 kcal, which means 8–10 million joules of energy.
A slice of bread has about 80 kcal, which is 335 kJ, while a banana has about 100 kcal, or 418 kJ.

Electrical Energy

Household energy consumption is measured in kilowatt-hours. If a 100-watt light bulb shines for 10 hours, its energy consumption is:

E = P × t = 0.1 kW × 10 h = 1 kWh

This equals 3.6 million joules of energy. A modern refrigerator consumes about 150 kWh of energy annually, which is about 540 million joules.

Physics and Scientific Measurements

In physical experiments, the joule is used to measure kinetic energy, heat, or electrical work. A 1 kg body falling from 1 meter height gains about 9.8 joules of kinetic energy due to gravity.

Training and Sports

During training, the human body consumes energy that transforms into heat. A 30-minute run burns about 300 kcal, or 1.25 million joules of energy. Sports equipment, such as smartwatches, often automatically calculate this value.

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Brief History of the Development of Energy Measurement

The concept of energy only received a precise scientific definition in the 19th century. In earlier centuries, heat and motion were considered different, independent phenomena. Joule's experiments proved that mechanical work can be transformed into heat – this led to the law of conservation of energy.

The calorie became widespread at the beginning of the 19th century, especially in the field of thermodynamics and nutritional science. The kilowatt-hour became indispensable with the advent of the electrical era in the 20th century, when energy consumption became mass-scale. The BTU spread during the industrial revolution in England and served to measure the efficiency of heating systems.

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Why Is It Important to Understand Energy Units?

Knowledge of energy conversions helps us correctly interpret our everyday environment and our decisions:

• Conscious consumption: We can understand how much energy we consume with a household appliance and how to reduce the electricity bill.
• Environmental awareness: By increasing energy efficiency, the ecological footprint can be reduced.
• Nutritional balance: Accurate interpretation of calories helps in creating a healthy diet.
• Scientific understanding: The relationships of different energy forms (mechanical, electrical, chemical, heat energy) form the basis of modern technology.

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Summary

Energy conversion is not just a mathematical operation, but a key to understanding the functioning of the universe. Joule, calorie, kilowatt-hour, and BTU all describe the same basic concept in different contexts: the flow and transformation of energy. Whether it's about nutrition, training, or the performance of a solar panel system, knowledge of energy units helps make more conscious decisions – and better understand how we utilize the world's resources.