Energy Converter

Convert between joules, calories, kilowatt-hours, BTU, electronvolts, and food Calories — for nutrition, electricity, heating, and physics.

Energy

J, cal, BTU, kWh

From

Result

0.001

1 J = 0.001 kJ

Popular conversions

What Is an Energy Converter?

An energy converter translates a quantity of energy between units — joules to calories, kilowatt-hours to BTU, electronvolts to joules. Energy is the capacity to do work, and the SI unit is the joule (J): the energy of one newton acting over one metre. Different fields adopted different units — nutrition uses Calories, electricity uses kilowatt-hours, heating uses BTU, physics uses electronvolts — but they all measure the same thing.

This converter routes through the joule with exact ratios. It clears up the most confusing case in measurement — the food 'Calorie' (capital C) is actually a kilocalorie, 1,000 of the small calories used in physics — and bridges the kilowatt-hour on your electricity bill to the joules and BTU of engineering.

This is one category of the full Unit Converter — pair it with our percentage calculator or scientific calculator for related everyday maths.

How Energy Conversion Works

Everything routes through the joule

Each unit has a fixed joule factor. The converter turns your input into joules, then projects it into every other energy unit so all outputs agree.

The food Calorie is a kilocalorie

A nutrition 'Calorie' (kcal) is 1,000 small calories. A 100-Calorie snack holds 100 kcal = 418,400 J — the source of endless confusion.

kWh links power and time

A kilowatt-hour is one kilowatt for one hour = 3,600,000 J (3.6 MJ). It's the standard billing unit for electrical energy.

BTU for heating and cooling

One BTU (British thermal unit) is 1,055 J — the heat to raise a pound of water 1 °F. Air conditioners and furnaces are rated in BTU.

Core Energy Conversion Factors

Multiply to reach joules; divide to come back. Watch the calorie-versus-Calorie distinction.

kcal → J

× 4184

One food Calorie (kilocalorie) is 4,184 joules. The small calorie is 4.184 J.

kWh → J

× 3,600,000

One kilowatt-hour is 3.6 million joules (3.6 MJ) — power times time.

BTU → J

× 1055.06

One British thermal unit is 1,055.06 joules, used for heating and cooling loads.

How to Use the Energy Converter

1
Enter the energy value
Type the amount of energy you want to convert — a food Calorie count, an electricity figure, a heat load.
2
Choose the 'from' unit
Pick joules, kilojoules, calories, Calories (kcal), kWh, BTU, or electronvolts.
3
Choose the 'to' unit
Select the target unit, or swap the two to reverse direction.
4
Read every unit at once
The all-units table shows the energy in joules, kcal, kWh, and BTU together for an instant cross-check.

Key Energy Concepts

Joule

The SI unit of energy: one newton-metre, or one watt-second. Every other energy unit is a fixed multiple of it.

Calorie vs Calorie

The small calorie (4.184 J) heats 1 g of water 1 °C. The food Calorie is 1,000 of those — a kilocalorie. Nutrition labels mean the big one.

Kilowatt-hour

Energy equal to one kilowatt sustained for one hour (3.6 MJ). The unit your power company bills, and the way battery and solar capacity is quoted.

Electronvolt

A tiny unit (1.602 × 10⁻¹⁹ J) used in atomic and particle physics — the energy an electron gains crossing one volt.

Real-World Energy Conversions

🍎

Food energy

A 250-Calorie snack is 250 kcal, or about 1,046 kJ — the kJ figure many countries print alongside Calories on labels.

💡

Electricity bill

Running a 1,000 W heater for 3 hours uses 3 kWh = 10.8 MJ. Bills quote kWh; engineering quotes joules.

❄️

Air conditioner rating

A 12,000 BTU/h AC unit (one 'ton' of cooling) moves about 3.5 kW of heat. AC capacity is rated in BTU per hour.

🔋

Battery capacity

A 100 Wh laptop battery holds 360,000 J. Device batteries are rated in watt-hours or milliamp-hours at a given voltage.

🔥

Heating fuel

A therm of natural gas is 100,000 BTU ≈ 105.5 MJ ≈ 29.3 kWh. Gas bills use therms or cubic metres of energy content.

⚛️

Particle physics

Chemical bonds are a few eV; a visible photon is ~2 eV. Electronvolts keep atomic energies on a human-readable scale.

Best Practices for Energy Conversion

✓Mind calorie vs Calorie. Nutrition 'Calories' are kilocalories (1,000 cal). Don't confuse them with the physics calorie, or food energy will be off by a factor of 1,000.
✓Use joules for physics, kWh for bills. Mechanics and thermodynamics expect joules; energy billing and storage use kWh. Convert at the boundary between the two worlds.
✓Separate energy from power. Energy (joules, kWh) is a total; power (watts) is a rate. A 'kW' and a 'kWh' are different quantities — don't convert one into the other.
✓Watch BTU vs BTU/h. BTU is energy; BTU per hour is power. AC and furnace ratings are usually BTU/h, a rate, not a total amount of energy.
✓Keep significant figures sensible. Food and bill figures are rounded estimates. Reporting a snack as 1,046.0 kJ implies a precision the label never had.

Common Energy Conversion Mistakes

Confusing calories and Calories

Treating a 200-Calorie food item as 200 small calories underestimates its energy 1,000-fold. Nutrition uses kilocalories.

Mixing up energy and power

kWh (energy) and kW (power) are different. A 2 kW heater isn't '2 kWh' — it uses 2 kWh only if run for one hour.

Reading BTU as BTU/h

Appliance ratings are usually BTU per hour (a rate). Treating that as total energy (BTU) drops the time dimension.

Forgetting kWh is 3.6 MJ

A kilowatt-hour is 3,600,000 joules, not 1,000. Using the wrong factor badly misstates energy in SI units.

Why Energy Conversion Matters

Energy is the common currency of food, fuel, electricity, and physics — yet each domain prices it in a different unit, so a single quantity wears many labels. The biggest practical trap is the food Calorie, a kilocalorie hiding behind a lower-case-sounding name, which makes nutrition math off by 1,000 if mishandled; the second is confusing energy (a total) with power (a rate).

Getting energy conversions right matters for diet tracking, electricity costs, heating and cooling sizing, and any engineering or science calculation. A converter that routes through the joule and exposes Calories, kWh, and BTU together keeps a dieter, an electrician, an HVAC installer, and a physicist all working from the same true quantity.

Built for dieters, electricians, HVAC installers, energy analysts, and physics students converting between joules, Calories, kWh, and BTU.

Linear unit factors follow the BIPM SI brochure, the NIST Guide to the SI, and ISO 80000. Currency rates load live from open.er-api.com; crypto prices from CoinGecko. See our methodology and editorial policy. Educational only — not certified for regulated trading, settlement, medical, or aerospace use.

Energy Converter FAQs

One food Calorie (with a capital C) is a kilocalorie, equal to 4,184 joules or 4.184 kilojoules. So a 100-Calorie snack holds about 418 kJ. The small calorie used in physics is just 4.184 J — 1,000 times smaller — which is the source of most energy-conversion confusion.

Multiply kilowatt-hours by 3,600,000 (3.6 million) to get joules, because one kWh is one kilowatt sustained for one hour: 1,000 W × 3,600 s = 3.6 MJ. So 5 kWh is 18 million joules. Megajoules (MJ) are often easier to read: 5 kWh = 18 MJ.

A lower-case calorie is the energy to heat one gram of water by 1 °C (4.184 J). An upper-case Calorie, used on food labels, is a kilocalorie — 1,000 small calories. Nutrition always means the big one, so 'a 500-Calorie meal' is 500 kcal, about 2,092 kJ.

One kilowatt-hour equals about 3,412 BTU, since 1 kWh = 3.6 MJ and 1 BTU = 1,055 J. This conversion comes up when comparing electric heating (kWh) with gas or AC systems rated in BTU. Note that appliance ratings are usually BTU per hour, a power, not total energy.

No. Energy (joules, kWh, Calories) is a total amount; power (watts, kW, BTU/h) is the rate energy is used. A 2 kW heater run for 3 hours uses 6 kWh of energy. Converting kW into kWh requires a time, so they are not directly interchangeable.

It uses standard definitions (1 kcal = 4,184 J, 1 kWh = 3.6 MJ, 1 BTU = 1,055.06 J, 1 eV = 1.602176634 × 10⁻¹⁹ J) and routes every conversion through the joule at full precision, so the result is exact to your input precision.