Energy & Power Converter

Quick Reference Table

From	To	Multiply By	Example
Joules	Calories	0.239006	1000 J = 239 cal
Calories	Joules	4.184	100 cal = 418.4 J
kWh	Joules	3,600,000	1 kWh = 3.6 MJ
kWh	BTU	3412.14	1 kWh = 3412 BTU
BTU	Joules	1055.06	1 BTU = 1055 J
Watt-hours	Joules	3600	1 Wh = 3600 J
Calories	Kilocalories	0.001	1000 cal = 1 kcal
eV	Joules	1.602 × 10⁻¹⁹	1 eV = 1.602×10⁻¹⁹ J

Energy Benchmarks

Description	Joules	Calories	kWh
Lifting 1 kg by 1 meter	9.8 J	2.3 cal	0.0000027 kWh
AA battery	~10,000 J	2,390 cal	0.0028 kWh
Candy bar	~1,000,000 J	239,000 cal	0.28 kWh
Daily human diet (2000 kcal)	8,368,000 J	2,000,000 cal	2.3 kWh
US household (daily avg)	~108 MJ	25.8 Mcal	30 kWh
Gallon of gasoline	~120 MJ	28.7 Mcal	33.4 kWh
Lightning bolt	~1 GJ	239 Mcal	278 kWh
Atomic bomb (Hiroshima)	~63 TJ	15 Tcal	17.5 GWh

Understanding Energy & Power Units

Joule (J)

Definition: The SI unit of energy. One joule equals the energy required to apply a force of one newton over a distance of one meter, or the energy dissipated as heat by one ampere of current flowing through one ohm of resistance for one second.

History: Named after English physicist James Prescott Joule (1818-1889) who studied the nature of heat and energy. Officially adopted as the SI unit of energy in 1889.

Current Use: Universal scientific standard for energy measurements. Used in physics, chemistry, engineering, and all scientific calculations. Common multiples: kilojoule (kJ = 1,000 J), megajoule (MJ = 1 million J), gigajoule (GJ = 1 billion J). Food energy labels in many countries show both calories and kilojoules.

Calorie (cal) & Kilocalorie (kcal)

Definition: One small calorie (cal) is the energy needed to raise the temperature of 1 gram of water by 1°C. One kilocalorie (kcal or Cal with capital C) equals 1,000 small calories. Food labels use kilocalories but often write "Calories" (capital C).

History: Introduced by French chemist Nicolas Clément in 1824. Became the standard unit for heat and nutrition before joules. The "food Calorie" (capital C) was established to avoid confusion with the small calorie.

Current Use: Predominantly used for food energy content worldwide. Nutrition labels show "Calories" (meaning kilocalories). Diet and fitness applications track calorie intake and burn. Still used in chemistry when discussing reactions and heat capacity. 1 kcal = 4,184 joules.

Kilowatt-hour (kWh)

Definition: The energy consumed by using one kilowatt (1,000 watts) of power continuously for one hour. Equal to 3.6 megajoules or 860 kilocalories. Not an SI unit but widely used for practical measurements.

History: Emerged with electric power industry in late 1800s. Provided a convenient way to measure and bill electricity consumption. More intuitive than joules for household energy use.

Current Use: Standard unit for electricity billing worldwide. Electric utility companies charge per kWh consumed. Used to rate appliance energy consumption, solar panel output, battery capacity (especially electric vehicles), and total household energy usage. Average US home uses 30 kWh per day.

Watt-hour (Wh)

Definition: One watt of power sustained for one hour. Equal to 3,600 joules. One kilowatt-hour equals 1,000 watt-hours.

History: Derived from the watt, named after Scottish inventor James Watt (1736-1819). Became standard with electrical systems.

Current Use: Used for smaller batteries and devices. Phone batteries (10-20 Wh), laptop batteries (50-100 Wh), portable power banks, and small solar installations. Electric vehicle batteries measured in kWh (Tesla Model 3: ~50-75 kWh).

British Thermal Unit (BTU)

Definition: The energy required to raise the temperature of one pound of water by one degree Fahrenheit. One BTU equals approximately 1,055 joules or 252 calories.

History: Developed in the 19th century alongside imperial measurements. Used extensively in British and American engineering before metrication.

Current Use: Still widely used in United States for heating and cooling systems (furnace ratings, air conditioner capacity), natural gas billing (therms = 100,000 BTU), and power plant output. HVAC systems rated in BTU/hour. Gradually being replaced by metric units internationally but remains standard in US HVAC industry.

Electronvolt (eV)

Definition: The energy gained by a single electron when accelerated through an electric potential difference of one volt. Equal to approximately 1.602 × 10⁻¹⁹ joules. Extremely tiny unit.

History: Emerged in early 20th century with atomic and particle physics research. Convenient for describing energy at atomic and subatomic scales.

Current Use: Standard unit in particle physics, atomic physics, and quantum mechanics. Used to describe particle energies in accelerators (Large Hadron Collider: trillions of eV), photon energies, ionization energies, and semiconductor band gaps. Common multiples: keV (thousand), MeV (million), GeV (billion), TeV (trillion electronvolts).

Erg

Definition: The CGS (centimeter-gram-second) unit of energy. One erg equals 10⁻⁷ joules. Very small unit rarely used today.

History: Part of the CGS system developed in 1873. Named from Greek "ergon" meaning work.

Current Use: Largely obsolete, replaced by joules. Occasionally appears in older scientific literature and astrophysics. 1 erg = 0.0000001 joules.

Therm

Definition: Equal to 100,000 BTU or approximately 105.5 megajoules. Used primarily for natural gas.

History: Introduced by US natural gas industry for billing convenience.

Current Use: Standard unit for natural gas billing in United States. Gas meters measure consumption in therms or CCF (hundred cubic feet, approximately 1 therm). One therm can heat ~10 gallons of water from cold to hot.

Foot-Pound (ft-lb)

Definition: The energy required to raise one pound against gravity for a distance of one foot. Equal to approximately 1.356 joules.

History: Imperial unit of energy used in engineering and mechanics.

Current Use: Used in US automotive industry for torque specifications (though torque is technically different from energy). Also appears in ballistics for measuring bullet energy. Mostly replaced by joules in scientific contexts.

Frequently Asked Questions

How many joules are in a calorie?

One small calorie (cal) equals exactly 4.184 joules. One kilocalorie (kcal or food "Calorie") equals 4,184 joules or 4.184 kilojoules. To convert calories to joules, multiply by 4.184. Example: 500 cal × 4.184 = 2,092 joules.

How many joules in a kWh?

One kilowatt-hour equals 3,600,000 joules (3.6 megajoules). Calculation: 1 kWh = 1,000 watts × 3,600 seconds = 3,600,000 joules. This is why kWh is more practical for electricity billing than joules.

What's the difference between calories and Calories in food?

Food labels use "Calories" (capital C), which are actually kilocalories (kcal). One food Calorie = 1,000 small calories (cal). So a 200-Calorie snack contains 200,000 small calories or 200 kcal. Scientists use small calories; nutritionists use food Calories (kcal).

How do I convert kWh to BTU?

Multiply kWh by 3,412.14. Formula: BTU = kWh × 3,412.14. Example: 10 kWh × 3,412.14 = 34,121 BTU. This conversion is common when comparing electric heating to gas heating efficiency.

What is a watt vs watt-hour?

A watt (W) measures power (energy per second), while a watt-hour (Wh) measures energy. Think of it like speed vs distance: watts are speed (mph), watt-hours are distance (miles). A 60W bulb running for 2 hours uses 120 Wh of energy. Power = energy ÷ time.

How many calories does the human body burn daily?

Average adult burns 1,800-2,500 food Calories (kcal) daily depending on size, age, sex, and activity level. Basal metabolic rate (BMR) for resting is ~1,400-1,800 kcal. This equals approximately 7.5-10.5 million joules or 2-3 kWh per day.

Why does my utility bill show kWh instead of joules?

kWh is more intuitive and practical than joules for household energy. 1 kWh is easier to understand than 3.6 million joules. Historical reasons: electric industry established kWh as the billing standard before SI units were universally adopted. The numbers are more manageable (30 kWh vs 108 million joules).

What is the most efficient energy unit?

No unit is inherently "more efficient" - they're just different ways to measure the same thing. However, joules are the SI standard for scientific work. For electricity, kWh is most practical. For food, Calories (kcal) are standard. Choose the unit that fits your application and region.

Common Uses for Energy Conversion

• Nutrition & Diet: Converting between calories and kilojoules on food labels
• Electricity Bills: Understanding kWh consumption and costs
• HVAC Systems: Converting BTU ratings to kW for air conditioners and heaters
• Solar Panels: Calculating energy production in kWh or joules
• Fitness Tracking: Converting exercise calories to energy expenditure
• Engineering: Scientific calculations requiring joules
• Physics Education: Understanding energy in different contexts
• Battery Capacity: Comparing Wh ratings across devices

Energy Conversion Tips

1. Remember key ratios: 1 cal = 4.184 J, 1 kWh = 3.6 MJ, 1 BTU ≈ 1,055 J
2. Food Calories are kcal: 100 Calories on a label = 100,000 small calories
3. kWh for electricity: Most practical for household energy discussions
4. BTU in US HVAC: Air conditioner capacity usually in BTU/hour
5. Use joules for science: Universal SI unit preferred in research
6. Battery capacity in Wh: Smaller devices; kWh for electric cars
7. Gas billing in therms: 1 therm = 100,000 BTU (US)

Interesting Energy Facts

• Human body at rest: ~80 watts (like a light bulb)
• Hummingbird metabolism: 10x human per gram of body weight
• Sun's output: 3.8 × 10²⁶ watts (386 billion billion megawatts)
• Atomic bomb (Hiroshima): ~15 kilotons TNT = 63 terajoules
• Annual US energy use: ~100 quadrillion BTU (100 quads)
• Lightning bolt: ~1 billion joules (could power 100W bulb for 115 days)
• Gallon of gas: ~120 MJ (equal to 33.4 kWh of electricity)
• Human brain: ~20 watts (20% of resting metabolism)
• World energy consumption: ~580 million terajoules annually
• Single AA battery: ~10,000 joules stored energy
• M&M candy: ~4 calories = 16.7 joules
• Climbing stairs (1 floor): ~2,000 joules (~0.5 Calories)

Energy in Different Contexts

Household Energy

Average US home uses 30 kWh/day (900 kWh/month). Refrigerator: 100-200W continuous (2.4-4.8 kWh/day). LED bulb: 10W (0.24 kWh/day if on 24 hours). Electric car charging: 50-75 kWh fills battery.

Food Energy

Carbohydrates: 4 Calories/gram. Protein: 4 Calories/gram. Fat: 9 Calories/gram. Alcohol: 7 Calories/gram. Marathon burns ~2,600 Calories. Walking 1 mile burns ~100 Calories.

Fuel Energy Density

Gasoline: 34 MJ/liter. Diesel: 38 MJ/liter. Natural gas: 38 MJ/m³. Coal: 24 MJ/kg. Lithium battery: 0.5-0.9 MJ/kg. TNT: 4.6 MJ/kg. Uranium-235: 88 million MJ/kg.

Renewable Energy

Solar panel (1m²): 150-200W peak. Wind turbine (large): 2-3 MW. Hydroelectric dam: varies widely, 100 MW-22,500 MW (Three Gorges Dam). Human pedaling: 75W sustained, 400W sprint.