Energy Converter
Convert joules, calories, kWh, BTU, ft-lb, and electronvolts.
About the Energy Converter
The energy converter translates between nine units of energy: joule (the SI unit), kilojoule, calorie, kilocalorie, watt-hour, kilowatt-hour, British thermal unit, foot-pound, and electronvolt. All conversions are expressed in joules as the pivot unit, with the factors derived from the standard definitions.
The joule is the SI derived unit of energy, defined as one newton-metre or one watt-second. It is named after James Prescott Joule, whose 1845 experiments demonstrated the mechanical equivalent of heat. Per the SI Brochure 9th edition, the joule is derived from the SI base units of kilogram, metre, and second as 1 kg m² / s².
The calorie was originally defined as the energy needed to raise 1 gram of water by 1°C. The modern "thermochemical calorie" is defined as exactly 4.184 joules, while the "international table calorie" is 4.1868 joules. This converter uses the thermochemical definition. The kilocalorie (1000 calories) is the unit used for food energy, where it is often misleadingly called "Calorie" with a capital C.
The kilowatt-hour is the unit used for electrical energy billing: 1 kWh = 1000 watts sustained for one hour = 3.6 million joules. The British thermal unit (BTU) is the imperial unit of energy, defined as the energy needed to raise 1 pound of water by 1°F, approximately 1055 joules. The electronvolt is the atomic-scale unit, equal to the energy gained by an electron moving across 1 volt, about 1.602 * 10^-19 joules.
How It Works
The converter uses the two-step method through joules:
base_in_joules = value * factor[from]
result = base_in_joules / factor[to]The factor table (in J) is:
J = 1 J kJ = 1,000 J cal = 4.184 J
kcal = 4,184 J Wh = 3,600 J kWh = 3,600,000 J
BTU = 1,055.06 J ft-lb = 1.35582 J eV = 1.602e-19 JThe metric factors (kJ, kWh) are exact. The thermochemical calorie is exactly 4.184 J. The watt-hour is exactly 3600 J (since 1 watt = 1 joule per second and 1 hour = 3600 seconds). The BTU is approximately 1055.06 J; the displayed value uses the international table BTU.
The foot-pound is approximately 1.35582 J, derived from the imperial definitions of the foot and the pound-force. The electronvolt is approximately 1.602 * 10^-19 J, derived from the elementary charge (exactly 1.602176634 * 10^-19 coulombs per the 2019 SI redefinition).
The two-step method through joules keeps the code simple. The pivot unit also matches the SI convention, which expresses energy in joules. The very small magnitude of the electronvolt (10^-19 J) compared to the very large magnitude of the kilowatt-hour (3.6 * 10^6 J) illustrates the wide range of scales that energy conversion must accommodate.
Worked Examples
Converting 1 kilowatt-hour to joules: the input in J is 1 * 3.6e6 = 3,600,000 J, displayed directly. This is the energy used by a 1000-watt appliance running for one hour. Converting to kilocalories gives 3,600,000 / 4184 = 860.42 kcal, which is roughly the energy in a typical fast-food meal.
Converting 1 BTU to joules: the input in J is 1055.06 J, displayed directly. Converting to watt-hours gives 1055.06 / 3600 = 0.293 Wh. The BTU is commonly used to rate the cooling capacity of air conditioners: a 10,000 BTU air conditioner uses about 2.93 kWh of energy per hour of operation.
Converting 1 electronvolt to joules: the input in J is 1 * 1.602e-19 = 1.602e-19 J, displayed in scientific notation. Converting to kilojoules gives 1.602e-22 kJ. The electronvolt is the unit used in particle physics: the rest energy of an electron is about 511,000 eV, or about 8.19e-14 J.
When to Use This Tool
Use the energy converter when you need to:
- Convert electricity bills from kilowatt-hours to joules for physics homework.
- Translate food energy labels between kilocalories and kilojoules.
- Convert HVAC specifications from BTUs to kilowatts for sizing.
- Translate mechanical work from foot-pounds to joules for engineering.
- Convert particle physics energies from electronvolts to joules.
- Verify that an exercise burn estimate matches the food energy label.
- Translate battery capacities from watt-hours to joules for design.
Limitations & Disclaimer
This converter uses standard conversion factors: the thermochemical calorie (4.184 J), the international table BTU (1055.06 J), and the elementary charge (1.602e-19 C). It does not handle the 15°C calorie, the international steam table calorie, or the therm (100,000 BTU). The electronvolt factor uses the 2019 SI-defined value of the elementary charge. Results are subject to IEEE 754 floating-point precision. See our disclaimer for full details.
Frequently Asked Questions
What is the difference between a calorie and a Calorie?
A calorie (lowercase) is the energy needed to raise 1 gram of water by 1°C, equal to about 4.184 joules. A Calorie (uppercase), also called a kilocalorie, is 1000 calories. Food labels in the US use the term 'Calorie' but mean the kilocalorie, which causes endless confusion. A typical adult needs about 2000 Calories (kilocalories) per day.
Why are there different definitions of the calorie?
Because the original definition depends on the starting temperature of the water (since the heat capacity of water varies slightly with temperature). The thermochemical calorie is defined as exactly 4.184 J for consistency. The international table calorie is 4.1868 J, and the 15°C calorie is 4.1855 J. This converter uses the thermochemical definition, the most common in chemistry.
What is a kilowatt-hour and why is it used for billing?
A kilowatt-hour (kWh) is the energy delivered by 1000 watts of power sustained for one hour, equal to 3.6 million joules. It is used for electricity billing because it produces convenient numbers: a typical household uses 10 to 30 kWh per day. The joule is too small for everyday use: 1 kWh equals 3,600,000 J.
What is a BTU and where is it used?
The British thermal unit is the energy needed to raise 1 pound of water by 1°F, equal to about 1055 joules. It is used in the US to rate the heating and cooling capacity of HVAC equipment. A typical home furnace is rated at 80,000 to 100,000 BTU per hour, and a window air conditioner at 5,000 to 10,000 BTU per hour.
What is an electronvolt and why is it so small?
An electronvolt is the energy gained by an electron when it moves across a potential difference of 1 volt, equal to about 1.602 * 10^-19 joules. It is used in particle physics because the energies involved are tiny on the macroscopic scale. The rest energy of an electron is about 511,000 eV, and the energies reached by the Large Hadron Collider are about 13 trillion eV.
Is energy the same as power?
No. Energy is the capacity to do work, measured in joules. Power is the rate at which energy is delivered, measured in watts (1 watt = 1 joule per second). A 100-watt light bulb consumes 100 joules per second; running it for 10 hours uses 1000 watt-hours = 1 kWh = 3.6 million joules of energy.
Last updated: September 9, 2026 · Author: HT99 Tools Editorial Team