Energy Converter – Convert Between Joules, Calories, Kilowatt-Hours, BTUs & More
Our free energy converter helps you make accurate conversions between different energy units used across various fields. Energy measurement varies between disciplines—physicists use joules, nutrition labels show calories, electricity bills list kilowatt-hours, and heating systems work with BTUs. This tool simplifies these conversions, whether you're working on scientific calculations, engineering problems, nutritional analysis, or understanding your utility bills.
Important Notes:
- Results are calculated using precise conversion factors and may be rounded for display purposes.
- The "calorie" used in food nutrition labels is actually a kilocalorie (kcal).
- Electronvolt (eV) is typically used for atomic and subatomic energy measurements.
- Different fields traditionally use different units: physics uses joules, nutrition uses calories, electricity uses kilowatt-hours, and heating/cooling uses BTUs.
- For scientific work requiring extreme precision, consult the latest official conversion standards.
Understanding Energy Units & Conversions
Energy is a fundamental concept that appears in virtually every scientific field, from physics and chemistry to biology and engineering. Because energy is used in so many different contexts, various units have been developed to express energy quantities in ways that are practical for specific applications.
What Is Energy?
Energy is the capacity to do work or produce heat. It exists in many forms—kinetic, potential, thermal, electrical, chemical, nuclear, and electromagnetic—but all forms of energy are interchangeable and can be transformed from one type to another. The law of conservation of energy states that energy cannot be created or destroyed, only transformed or transferred.
SI and Metric Energy Units
- Joule (J): The standard SI unit of energy
- Kilojoule (kJ): 1,000 joules, common in physics
- Megajoule (MJ): 1,000,000 joules, used for larger energy quantities
- Calorie (cal): Amount of energy needed to raise 1g of water by 1°C
- Kilocalorie (kcal): 1,000 calories, used in nutrition as "Calorie" (with capital C)
- Watt-hour (Wh): Energy of one watt sustained for one hour
- Kilowatt-hour (kWh): 1,000 watt-hours, used in electrical billing
Non-SI and Specialized Energy Units
- British Thermal Unit (BTU): Energy to raise 1 pound of water by 1°F
- Foot-pound (ft-lbf): Energy to move 1 pound force through 1 foot
- Electronvolt (eV): Energy gained by an electron moving across 1 volt
- Therm: 100,000 BTU, used in natural gas metering
- Quad: Quadrillion (10^15) BTU, used for national energy statistics
- Ton of TNT: 4.184 gigajoules, used for explosions and seismic events
- Barrel of oil equivalent (BOE): ~6.1 gigajoules, used in energy industries
Key Energy Conversion Factors
Understanding the relationships between energy units is essential for accurate conversions. Here are the primary conversion factors that our calculator uses:
| Conversion | Exact Value | Approximate Value |
|---|---|---|
| 1 Joule (J) = | 1 J | Base unit |
| 1 Kilojoule (kJ) = | 1,000 J | 103 joules |
| 1 Calorie (cal) = | 4.184 J | ~4.2 joules |
| 1 Kilocalorie (kcal) = | 4,184 J | ~4.2 kilojoules |
| 1 Watt-hour (Wh) = | 3,600 J | 3.6 kilojoules |
| 1 Kilowatt-hour (kWh) = | 3,600,000 J | 3.6 megajoules |
| 1 British Thermal Unit (BTU) = | 1,055.06 J | ~1.055 kilojoules |
| 1 Electronvolt (eV) = | 1.602176634 × 10-19 J | Extremely small energy unit |
| 1 Foot-pound (ft-lbf) = | 1.3558179483314 J | ~1.36 joules |
Common Energy Units by Field
Physics & Scientific Research
- Joules (J): The standard SI unit, used in all physics equations
- Ergs: Used in CGS system (1 erg = 10-7 joules)
- Electronvolts (eV): For atomic and particle physics
- Hartree: Atomic unit of energy used in quantum chemistry
- Rydberg: Used for energy levels in spectroscopy
Note: Scientific papers almost exclusively use joules or related SI units.
Food Science & Nutrition
- Kilocalories (kcal): Listed as "Calories" on food packaging
- Kilojoules (kJ): Required on food labels in many countries alongside kcal
- Megajoules (MJ): Used for daily dietary energy requirements
Example: A 2,000 Calorie diet equals 2,000 kcal or approximately 8.37 MJ per day.
Electricity & Power Generation
- Kilowatt-hours (kWh): Standard for electrical billing
- Megawatt-hours (MWh): Used for power station output
- Gigawatt-hours (GWh): For grid-scale electricity
- Watt-seconds: Equivalent to joules, used in some electrical contexts
Example: The average US household uses about 893 kWh of electricity per month.
Heating, Cooling & HVAC
- British Thermal Units (BTU): Standard for heating/cooling capacity
- Therms: Used for natural gas billing (1 therm = 100,000 BTU)
- Tons of refrigeration: For cooling capacity (1 ton = 12,000 BTU/h)
- Millions of BTU (MMBTU): Used in industrial heating
Example: A typical home air conditioner might be rated at 18,000 BTU (1.5 tons).
Mechanics & Engineering
- Foot-pounds (ft-lbf): Used in US mechanical engineering
- Newton-meters (N·m): The SI unit of torque (equivalent to joules)
- Horsepower-hours: Used for mechanical work over time
- Kilogram-force meters: Used in some mechanical applications
Example: A torque wrench might be calibrated in both N·m and ft-lbf.
Energy Industry & Economics
- Barrel of oil equivalent (BOE): ~6.1 GJ, standard in oil industry
- Ton of coal equivalent (TCE): ~29.3 GJ, used for comparing energy sources
- Quads: Quadrillion BTU, used in national energy statistics
- Terawatt-hours (TWh): Used for national electricity generation
- Exajoules (EJ): Used for global energy consumption (1 EJ = 1018 J)
Example: Global annual energy consumption is around 600 exajoules.
Frequently Asked Questions About Energy Conversion
How do I convert joules to kilowatt-hours or calories?
To convert joules to kilowatt-hours, divide the number of joules by 3,600,000 (since 1 kWh = 3.6 million joules). For example, 1,000,000 joules ÷ 3,600,000 = 0.278 kWh.
To convert joules to calories, divide by 4.184 (since 1 calorie = 4.184 joules). For example, 1,000 joules ÷ 4.184 = 239.01 calories.
Our calculator handles these conversions automatically, providing precise results without having to remember conversion factors.
What's the difference between a calorie (cal) and a kilocalorie (kcal)?
A calorie (cal) is defined as the energy needed to raise the temperature of 1 gram of water by 1°C. A kilocalorie (kcal) is 1,000 calories—the energy needed to raise 1 kilogram of water by 1°C.
The confusion arises because in nutrition and food labeling, what's labeled as "Calories" (with a capital C) are actually kilocalories. So when a food package says "200 Calories," it means 200 kcal or 200,000 cal.
This distinction is important for accuracy, especially when working with scientific calculations versus everyday nutritional information.
What's the difference between kcal and kWh?
While both are units of energy, they are used in different contexts and have different magnitudes:
- Kilocalorie (kcal): Used primarily in nutrition and food science. 1 kcal = 4,184 joules.
- Kilowatt-hour (kWh): Used primarily for electrical energy, especially in utility billing. 1 kWh = 3,600,000 joules.
So 1 kWh equals approximately 860 kcal. For perspective, the energy in 1 kWh could power a 1000W microwave for one hour, while 860 kcal represents about 43% of a typical 2,000 Calorie daily diet.
What is a BTU and when is it used?
A BTU (British Thermal Unit) is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. One BTU equals approximately 1,055 joules.
BTUs are primarily used in:
- Heating and cooling systems (air conditioners, furnaces, heaters)
- Natural gas and other fuel measurement
- Industrial heating processes
- Building energy efficiency calculations
For example, air conditioners are often rated in BTUs per hour, with a typical home unit ranging from 5,000 to 24,000 BTU/h.
Can I use this for nutrition and electricity calculations?
Yes, this calculator is designed to handle energy conversions across multiple fields, including both nutrition and electricity:
- For nutrition: You can convert between calories (cal), kilocalories (kcal), joules (J), and kilojoules (kJ), which are all commonly used in nutritional contexts.
- For electricity: You can convert between watt-hours (Wh), kilowatt-hours (kWh), joules (J), and other energy units to understand electrical consumption or generation.
This makes the calculator valuable for dietitians, engineers, students, and anyone working across multiple fields that use different energy units.
How accurate are these energy conversions?
The conversions in this calculator use precise, standardized conversion factors based on the International System of Units (SI) and other recognized standards. For most practical applications, these convers