Energy Converter

Convert between different energy units - joules, calories, kilowatt-hours, BTU, and more.

Energy Converter

Value

From

Quick Values

1 Kilocalories =

4,184

Joules (J)

All Conversions

Joules4,184

Kilojoules4.184

Calories1,000

Kilocalories1

Watt-hours1.162222

Kilowatt-hours0.001162

Electron-volts2.6114e+22

BTU3.965651

Therms3.9657e-5

Foot-pounds3,085.960033

Ergs4.1840e+10

Megajoules0.004184

Common Conversions

1 calorie = 4.184 joules
1 kilocalorie (food calorie) = 4,184 joules
1 kWh = 3,600,000 joules
1 BTU = 1,055 joules
1 therm = 100,000 BTU

What is Energy Conversion?

Energy conversion transforms energy measurements between different units used in physics, engineering, nutrition, and everyday applications. Energy is the capacity to do work, and various fields have developed specialized units.

Unit	Symbol	Primary Use	Joules Equivalent
Joule	J	SI standard	1
Kilowatt-hour	kWh	Electricity billing	3,600,000
Calorie (small)	cal	Chemistry	4.184
Calorie (food)	kcal/Cal	Nutrition	4,184
British Thermal Unit	BTU	HVAC, heating	1,055
Electronvolt	eV	Particle physics	1.602 × 10⁻¹⁹
Foot-pound	ft·lb	Mechanical work	1.356

Energy Unit Relationships

1 kWh = 3.6 MJ = 3,412 BTU = 860 kcal

Where:

kWh= Kilowatt-hour
MJ= Megajoules
BTU= British Thermal Units
kcal= Kilocalories (food Calories)

Joules and the SI System

The joule (J) is the SI unit of energy, named after physicist James Prescott Joule. It's defined as the energy transferred when a force of one newton moves an object one meter.

Unit	Value	Typical Application
Nanojoule (nJ)	10⁻⁹ J	Electronics, sensors
Microjoule (µJ)	10⁻⁶ J	Lasers, medical devices
Millijoule (mJ)	10⁻³ J	Camera flashes
Joule (J)	1 J	Small mechanical work
Kilojoule (kJ)	10³ J	Food energy, chemistry
Megajoule (MJ)	10⁶ J	Industrial processes
Gigajoule (GJ)	10⁹ J	Energy production

Joule Definition

1 J = 1 N·m = 1 kg·m²/s² = 1 W·s

Where:

J= Joule
N·m= Newton-meter
W·s= Watt-second

Calories in Nutrition

The calorie has a confusing dual definition that often causes confusion. Understanding the difference is crucial for nutrition and science.

Type	Symbol	Definition	Joules
Small calorie	cal	Energy to heat 1g water by 1°C	4.184 J
Large Calorie (kilocalorie)	Cal, kcal	Energy to heat 1kg water by 1°C	4,184 J
Thermochemical calorie	calₜₕ	Defined as exactly 4.184 J	4.184 J

Important: Food labels use Calories (capital C) or kcal, which equals 1,000 small calories. A food item with "200 Calories" contains 200,000 small calories or 200 kcal.

Calorie Conversions

1 kcal (food Calorie) = 1,000 cal = 4,184 J = 4.184 kJ

Where:

kcal= Kilocalorie (food Calorie)
cal= Small calorie
kJ= Kilojoules

Electrical Energy Units

Electrical energy is typically measured in kilowatt-hours (kWh) for billing purposes, representing the energy used by a 1,000-watt appliance running for one hour.

Unit	Definition	Joules	Use Case
Watt-hour (Wh)	1 W × 1 hour	3,600 J	Small electronics
Kilowatt-hour (kWh)	1 kW × 1 hour	3.6 MJ	Household electricity
Megawatt-hour (MWh)	1 MW × 1 hour	3.6 GJ	Commercial/industrial
Gigawatt-hour (GWh)	1 GW × 1 hour	3.6 TJ	Power plant output
Terawatt-hour (TWh)	1 TW × 1 hour	3.6 PJ	National energy stats

Example: If electricity costs $0.12/kWh, running a 100W light bulb for 10 hours costs: 0.1 kW × 10 hours × $0.12 = $0.12.

Electrical Energy

Energy (kWh) = Power (kW) × Time (hours)

Where:

E= Energy in kilowatt-hours
P= Power in kilowatts
t= Time in hours

BTU and Thermal Energy

The British Thermal Unit (BTU) is commonly used in the United States for heating, cooling, and HVAC systems. It represents the energy needed to raise one pound of water by one degree Fahrenheit.

Application	Typical BTU Rating	kWh Equivalent
Window AC unit	5,000-12,000 BTU/hr	1.5-3.5 kW
Central AC (3 ton)	36,000 BTU/hr	10.5 kW
Gas furnace	60,000-120,000 BTU/hr	17.5-35 kW
Water heater	30,000-50,000 BTU/hr	8.8-14.6 kW
Gas stove burner	7,000-12,000 BTU/hr	2-3.5 kW

BTU Conversions

1 BTU = 1,055 J = 252 cal = 0.293 Wh

Where:

BTU= British Thermal Unit
J= Joules
Wh= Watt-hours

Scientific Energy Units

Scientific fields often use specialized energy units appropriate to their scale of measurement:

Unit	Value	Field	Example Use
Electronvolt (eV)	1.602 × 10⁻¹⁹ J	Particle physics	Particle accelerators
Erg	10⁻⁷ J	CGS system	Older scientific literature
Hartree	4.36 × 10⁻¹⁸ J	Atomic physics	Quantum chemistry
Rydberg	2.18 × 10⁻¹⁸ J	Spectroscopy	Atomic energy levels
Ton of TNT	4.184 GJ	Explosive energy	Comparing explosions

The electronvolt is particularly useful because particle energies are often in the keV to TeV range.

Electronvolt Scales

1 keV = 10³ eV, 1 MeV = 10⁶ eV, 1 GeV = 10⁹ eV, 1 TeV = 10¹² eV

Where:

eV= Electronvolt
keV= Kiloelectronvolt
MeV= Megaelectronvolt
GeV= Gigaelectronvolt

Energy Content of Fuels

Different fuels have varying energy densities, measured as energy per unit mass or volume:

Fuel	Energy Density (MJ/kg)	Energy Density (kWh/kg)
Hydrogen	142	39.4
Natural gas	55	15.3
Gasoline	46	12.8
Diesel	45	12.5
Coal (anthracite)	32	8.9
Wood (dry)	18	5.0
Lithium-ion battery	0.5-0.9	0.14-0.25

Note that batteries have much lower energy density than fuels, explaining why electric vehicle range remains a challenge despite efficiency advantages.

Worked Examples

Convert Food Calories to Kilojoules

Problem:

A meal contains 750 Calories. How many kilojoules is this?

Solution Steps:

1Recognize that food Calories = kilocalories (kcal)
21 kcal = 4.184 kJ
3750 kcal × 4.184 kJ/kcal = 3,138 kJ
4Round: approximately 3,140 kJ

Result:

750 Calories = 3,138 kJ (approximately 3.14 MJ)

Calculate Electricity Cost

Problem:

A 1,500W space heater runs for 8 hours. At $0.15/kWh, what is the energy use and cost?

Solution Steps:

1Convert watts to kilowatts: 1,500 W = 1.5 kW
2Calculate energy: 1.5 kW × 8 hours = 12 kWh
3Calculate joules: 12 kWh × 3.6 MJ/kWh = 43.2 MJ
4Calculate cost: 12 kWh × $0.15 = $1.80

Result:

Energy used: 12 kWh (43.2 MJ). Cost: $1.80

Convert BTU to Other Units

Problem:

An air conditioner is rated at 12,000 BTU/hr. Convert to watts and kJ/hr.

Solution Steps:

1BTU to watts: 12,000 BTU/hr × 0.293 W/(BTU/hr) = 3,516 W
2Or: 12,000 BTU/hr ÷ 3.412 = 3,517 W ≈ 3.5 kW
3BTU to kJ: 12,000 BTU × 1.055 kJ/BTU = 12,660 kJ/hr
4This is a standard 1-ton AC unit (1 ton = 12,000 BTU/hr)

Result:

12,000 BTU/hr = 3,516 watts = 12,660 kJ/hr

Tips & Best Practices

✓Food Calories = kilocalories (kcal) = 4.184 kilojoules (kJ)
✓1 kWh = 3.6 MJ = 3,412 BTU - memorize this for electricity calculations
✓For HVAC: 1 ton of cooling = 12,000 BTU/hr ≈ 3.5 kW
✓To estimate annual electricity cost: (watts × hours/day × 365) ÷ 1000 × rate
✓BTU ≈ kJ (multiply BTU by ~1 for quick kJ estimate)
✓International food labels use kJ; US labels use Calories (divide kJ by 4.2)
✓Energy density of gasoline (~34 MJ/L) explains why batteries can't match fuel tank range

Frequently Asked Questions

The small calorie (cal) is the energy needed to raise 1 gram of water by 1°C, equal to 4.184 joules. The large Calorie (Cal), also written as kcal or kilocalorie, equals 1,000 small calories. Food labels use Calories (capital C) or kcal, so when you see '200 Calories' on a food package, it means 200 kilocalories or 200,000 small calories. This confusing dual naming convention persists for historical reasons.

One kilowatt-hour equals exactly 3,600,000 joules (3.6 MJ). This is calculated as: 1 kW × 1 hour = 1,000 watts × 3,600 seconds = 3,600,000 watt-seconds = 3,600,000 joules. The kilowatt-hour is preferred for electricity billing because the numbers are more manageable than millions of joules.

BTU (British Thermal Unit) ratings are traditional in the US HVAC industry because the system developed before metric standardization. One BTU is the energy to raise one pound of water by one degree Fahrenheit—a practical measure for heating applications. AC units are often rated in BTU/hr or 'tons' (1 ton = 12,000 BTU/hr, the cooling capacity to freeze one ton of ice per day). While kW is more universal, BTU persists due to industry convention.

Key conversions to memorize: 1 kWh = 3,412 BTU = 3.6 MJ. For calories: 1 kcal = 4.184 kJ. For quick estimates: BTU to kJ, multiply by about 1.055; kcal to kJ, multiply by about 4.2. Most scientific work uses joules (SI), while US residential applications use kWh for electricity and BTU for heating/cooling.

An electronvolt (eV) is the energy gained by an electron accelerating through a 1-volt potential difference, equal to 1.602 × 10⁻¹⁹ joules. It's used in particle physics, atomic physics, and chemistry because joule values would be inconveniently small (billions of decimal places). For example, visible light photons have energies around 1.5-3 eV, and the Large Hadron Collider operates at 6.5 TeV (6.5 trillion eV) per beam.

Find the appliance's wattage (usually on a label or manual), then multiply by hours of use: Energy (kWh) = Watts ÷ 1000 × Hours. For example, a 100W light bulb on for 10 hours uses 100 ÷ 1000 × 10 = 1 kWh. Multiply by your electricity rate (e.g., $0.12/kWh) to find the cost. For appliances that cycle on/off (like refrigerators), actual consumption is lower than the rated wattage.

Sources & References

NIST - SI Units: Energy (2024)
U.S. Energy Information Administration (2024)
BIPM - International System of Units (2024)
ASHRAE - HVAC Fundamentals (2024)

Last updated: 2026-01-22

Energy Converter