What is energy?

Energy is the capacity to do work – move objects, heat substances, light bulbs, run machines. It’s measured in many different units across different contexts: joules (J) is the SI scientific unit, calories for food and nutrition, kilowatt-hours (kWh) on your electricity bill, BTU for heating and cooling systems, foot-pounds for mechanical engineering, electron-volts (eV) for particle physics. All these measure the same physical quantity but at vastly different scales. A AAA battery holds about 4,500 J (1.25 Wh). Burning a candle for an hour uses ~360,000 J. A monthly Indian household uses ~300 kWh = ~1 billion joules. This converter lets you translate between any energy units, useful for engineering, physics homework, energy audits, nutrition tracking, and understanding energy bills.

How to use this tool

1. Enter value — The energy amount you want to convert.
2. Select ‘from’ unit — Joules (J), kJ, calories, kcal, watt-hours, kWh, BTU, electron-volts, or foot-pounds.
3. Read all conversions — Every unit shows the equivalent. Easy to see ‘1 kcal of food = 4,184 joules of mechanical work’.

Energy unit relationships

All conversions go via joules (SI base):

• 1 kJ = 1,000 J
• 1 calorie (cal) = 4.184 J (scientific calorie)
• 1 kilocalorie (kcal) = 4,184 J (food ‘Calorie’ with capital C)
• 1 watt-hour (Wh) = 3,600 J
• 1 kilowatt-hour (kWh) = 3,600,000 J = 3.6 MJ
• 1 BTU = 1,055.06 J (British Thermal Unit, used in HVAC)
• 1 electron-volt (eV) = 1.602 × 10^-19 J (particle physics)
• 1 foot-pound (ft-lb) = 1.35582 J (mechanical engineering, US)

Key conversions to memorize:

• 1 kcal (food calorie) = ~4.2 kJ
• 1 kWh (electricity) = 3.6 MJ = 860 kcal
• 1 BTU = ~1 kJ (close approximation)

Examples

• 1 banana (~100 kcal): 418,400 J, 116 Wh, 0.116 kWh, 396 BTU
• 10,000 steps (~400 kcal burned): 1.67 MJ, 464 Wh
• Monthly Indian home electricity (300 kWh): 1.08 GJ, 258,300 kcal, 1.024 million BTU
• AAA battery (~4,500 J): 1.07 kcal, 1.25 Wh, 4.27 BTU
• Burning 1 liter petrol (~32 MJ): 7,650 kcal, 8.89 kWh, 30,300 BTU
• 1 ton of TNT exploding: 4.184 GJ – hence atomic bomb energy in kilotons or megatons of TNT equivalent

Tips & best practices

• Food ‘Calorie’ (capital C) = 1 kilocalorie = 1,000 small calories. Confusing but standard in nutrition labels
• Electricity bill in kWh – multiply by your tariff rate to get cost
• Heating/cooling capacity often in BTU/h (BTU per hour) – power, not energy. 12,000 BTU/h ≈ 1 ton refrigeration ≈ 3.5 kW
• Battery capacity often in Wh or mAh. mAh needs multiplication by voltage for energy: 3000 mAh × 3.7V = 11.1 Wh
• eV (electron-volt) used at atomic scale – useless for everyday but standard in chemistry, particle physics
• Energy efficiency comparison: kJ/L for cars, kJ/serving for food, kWh/year for appliances
• 1 calorie ≠ 1 joule – common mistake. 1 kcal of food = 4,184 J of physical work potential

Limitations & notes

Conversion factors are exact (defined values). Calorie has two meanings – the calculator uses the modern scientific value (1 cal = 4.184 J). Old definitions varied slightly. For specialized energy units (erg = 10^-7 J, Hartree for atomic physics, kt for nuclear yields), use scientific tools. Caloric content of food is approximate measurement, not exact – real food has 5-20% variability.

Frequently Asked Questions

Why is food ‘Calorie’ different from physics ‘calorie’?

Food ‘Calorie’ (capital C) = 1 kilocalorie = 1,000 small calories. This convention emerged because the small calorie is too tiny for food labels. Modern labels increasingly show kJ alongside kcal. Most countries (EU, India) require both. US labels typically only show ‘Calories’ which means kcal.

How do I convert my electricity bill kWh to joules?

1 kWh = 3,600,000 J = 3.6 MJ. Monthly bill of 300 kWh = 1.08 gigajoules. Knowing this matters for energy efficiency comparisons – converting solar panel output (kWh) to home heating needs (often in MJ or BTU).

Why do air conditioners use BTU?

Historical reasons. BTU was the standard heat measurement in HVAC industry, originally British/American engineering. Modern AC units worldwide still use BTU/h ratings. 1 ‘ton’ of refrigeration = 12,000 BTU/h = 3.516 kW = the cooling power needed to freeze 1 ton (2,000 lb) of water in 24 hours.

How much energy is in 1 gram of food?

Depends on macronutrient: 1g of fat = 9 kcal = 37.7 kJ. 1g of carb or protein = 4 kcal = 16.7 kJ. 1g of alcohol = 7 kcal = 29.3 kJ. Pure water has no calories (no energy).

What’s electron-volt used for?

Atomic and particle physics. 1 eV is tiny – the energy a single electron gains across 1 volt. Chemical bonds are 0.1-10 eV. Visible light photons are 1.5-3 eV. X-rays are kiloelectron-volts (keV). Particle accelerators use TeV (teraelectron-volts). Too small for everyday use but essential in physics.

Why is kinetic energy of car so large?

Kinetic energy = (1/2) × m × v². Doubling speed quadruples energy! A 1500 kg car at 60 km/h has 208 kJ kinetic energy. At 120 km/h: 833 kJ (4x more). This is why high-speed crashes are exponentially more devastating – all that energy needs dissipation.

How much energy does my phone battery store?

Typical smartphone: 3,000-5,000 mAh at 3.7V. Convert: 4,000 mAh × 3.7V = 14.8 Wh = 53.3 kJ. Equivalent to about 13 kcal (1/4 banana of food energy). Surprisingly little energy by food standards, but enough for a day of phone use.

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