Energy Converter – Convert Joules, Calories, kWh Online
Convert between any energy unit with our free online energy converter. Supports joules, calories, kilocalories, kilowatt-hours, BTU, electron volts, and more for science, engineering, and nutrition calculations.
Understanding Energy Units
Energy comes in many forms and is measured in many units. The joule is the SI unit, defined as the work done by a force of one newton moving an object one meter. But different fields use different units based on what makes sense for their applications.
Nutrition uses Calories (kilocalories). Electricity uses kilowatt-hours. Heating uses BTUs. Physics uses electron volts for atomic-scale energy. Understanding how to convert between them is essential for comparing energy across contexts – like figuring out how many calories in a snack equals the energy to charge your phone.
Common Energy Units Explained
SI Units
Nutrition Units
Electrical Units
Heating Units
Scientific Units
Large Scale Units
Worked Examples
Example 1: 1 kWh to Joules
From: 1 kilowatt-hour
To: Joules
Result: 3,600,000 J (3.6 MJ)
One kWh is the energy of a 1000W appliance running for 1 hour. That's 3.6 million joules – enough to lift a car about 360 meters.
Example 2: 100 Calories to Joules
From: 100 calories (small c)
To: Joules
Result: 418.4 J
Note: Food labels use Calories (kcal). 100 food Calories = 100,000 cal = 418,400 J. That's the energy in about 1/4 of a banana.
Example 3: 1 BTU to Joules
From: 1 BTU
To: Joules
Result: 1,055.06 J
A typical window AC unit is rated in BTUs. A 10,000 BTU unit removes about 10.5 million joules of heat per hour.
Example 4: Daily Calorie Intake to Kilojoules
From: 2000 kilocalories (food Calories)
To: Kilojoules
Result: 8,368 kJ
The recommended 2000 Cal/day diet equals about 8,368 kJ. Australia and other countries list energy in kJ on food labels.
Example 5: 1 Joule to Electron Volts
From: 1 joule
To: Electron volts
Result: 6.242 × 10¹⁸ eV
The electron volt is tiny – it takes over 6 quintillion eV to make 1 joule. Particle physicists use eV because atomic energies are so small.
Example 6: 1 Therm to kWh
From: 1 therm
To: Kilowatt-hours
Result: 29.3 kWh
Natural gas is billed in therms. One therm equals about 29 kWh – enough to power an average home for a day.
Quick Fact
The Calorie on food labels is actually a kilocalorie. When a candy bar says "250 Calories," it means 250 kilocalories or 250,000 calories (small c). This convention started in the early 1900s to avoid writing large numbers. The capital C distinguishes food Calories from scientific calories.
Frequently Asked Questions
What's the difference between calorie and Calorie?
A calorie (lowercase c) is the energy to heat 1 gram of water by 1°C. A Calorie (uppercase C, also called kilocalorie) is 1,000 calories. Food labels use Calories. 1 Cal = 1 kcal = 4,184 J.
Why do electric bills use kWh instead of joules?
Kilowatt-hours are more practical for household energy. One joule is tiny – a 100W bulb uses 100 joules every second. kWh gives manageable numbers: that same bulb uses 0.1 kWh per hour.
How much energy is in a gallon of gasoline?
About 120-125 million joules (120 MJ) or 33 kWh. That's roughly 1,300 food Calories per gallon. A typical car gets 25-30 miles per gallon, so each mile costs about 4-5 Calories of gasoline energy.
What's a quad used for?
Quads measure national or global energy consumption. The US uses about 100 quads per year. One quad equals 1 quadrillion (10¹⁵) BTU or about 293 terawatt-hours.
Why do physicists use electron volts?
Atomic and particle energies are tiny in joules. An electron accelerated through 1 volt gains 1 eV = 1.6 × 10⁻¹⁹ J. Using eV avoids writing lots of zeros. Particle masses are even given in eV/c².
How do I compare energy costs across fuels?
Convert everything to the same unit (like kWh or MJ), then divide price by energy content. Natural gas is often cheapest per unit energy, followed by electricity, then gasoline.
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