Energy Unit Converter

How to use this calculator

Energy value — enter your number. The panel in the screenshot shows 1 joule converting to kilowatt-hours: 0.000000278 kWh, which is the number most people come here wanting to confirm.

From unit — the unit your value is in. The dropdown includes joules (J), kilojoules (kJ), calories (cal), kilocalories (kcal/Cal), watt-hours (Wh), kilowatt-hours (kWh), BTU, electronvolts (eV), and therms.

To unit — the unit you want as the primary output.

The output panel shows reference values across the main unit families simultaneously. Everything in the panel is the same quantity of energy, just labeled differently.

The real-world comparison section below the calculator maps your value against anchors like a slice of bread or a lightning bolt. That’s where the numbers stop being abstract.

Why energy has so many units

Most physical quantities have one or two units in common use. Energy somehow collected nine of them.

Part of that is historical. The joule was formalized by the International System in 1960, but BTU dates to the 1800s. Calorie predates it. These systems developed independently for different practical problems (steam engines, food science, electrical billing) and got calcified into their respective industries before anyone could unify them.

Part of it is genuinely useful variation. A joule is the right size for physics calculations but absurdly small for talking about food (an apple is 350,000 joules) and absurdly large for talking about electrons (a visible-light photon is about 3 × 10⁻¹⁹ joules). The electronvolt exists because 3 × 10⁻¹⁹ is an obnoxious number to type, not because anyone invented a new physics.

So the units stayed. And engineers kept needing to move between them.

The conversion structure

Energy units fall into loose families, and the tricky conversions are almost always cross-family.

The joule family scales by powers of 1,000:

• 1 kilojoule = 1,000 joules
• 1 megajoule = 1,000,000 joules

The calorie/kilocalorie family is small but confusing because of the capital-C problem (more on that below):

• 1 kilocalorie = 1,000 calories
• 1 calorie (small c) = 4.184 joules
• 1 kilocalorie = 4,184 joules

The electrical family connects to joules through time:

• 1 watt-hour = 3,600 joules (1W × 3,600 seconds)
• 1 kilowatt-hour = 3,600,000 joules

The thermal family comes from British engineering:

• 1 BTU = 1,055 joules (roughly)
• 1 therm = 100,000 BTU = 105,500,000 joules

The atomic family goes the other direction:

• 1 electronvolt = 1.602 × 10⁻¹⁹ joules

The two numbers worth memorizing: 3,600 (joules per watt-hour, same logic as the mAh-to-coulomb conversion) and 4,184 (joules per kilocalorie). Everything else you can derive or look up.

Full unit reference table

The kWh is the workhorse unit for anyone dealing with electricity. Everything from a phone charger (5–20 Wh per session) to a home’s monthly bill (500–1,000 kWh) to a utility-scale battery (hundreds of MWh) uses it.

The calorie confusion: small-c vs capital-C

This trips people up more than any other unit conversion.

A small-c calorie (cal) is the amount of heat needed to raise 1 gram of water by 1°C. It equals 4.184 joules. Chemists use it.

A capital-C Calorie (Cal) or kilocalorie (kcal) is 1,000 of those. It equals 4,184 joules. This is what food labels mean. When a granola bar says “200 Calories,” it means 200 kcal = 836,800 joules.

The practical result: if you see “Calories” on food, it’s kilocalories. If you see “calories” in a chemistry context, it’s small-c calories. The factor between them is exactly 1,000 and the only way to not confuse yourself is to just check which system you’re in.

The converter uses kcal for food-type conversions and cal for the small-c version. They’re separate entries in the dropdown.

Common conversions at a glance

Cross-family (the ones people actually need)

Within the joule family

The cross-family table is the one that matters. Within any single family it’s just powers of 1,000.

Real-world energy anchors

The output panel maps your value against real-world references. Some calibration points worth knowing:

The lightning bolt is the one that always surprises people. 30,000 amperes sounds enormous. But the flash lasts 0.2 milliseconds, so the total energy is closer to 1–5 GJ. A gallon of gasoline stores more usable energy, though obviously in a very different form.

Real-world examples

Electricity bill: kWh to joules

A home uses 800 kWh in a month. What’s that in joules?

800 kWh × 3,600,000 = 2,880,000,000,000 J (2.88 TJ)

In kilocalories: 2,880,000,000,000 / 4,184 = 688,000,000 kcal

In BTU: 800 × 3,412 = 2,729,600 BTU

None of those numbers are intuitive. That’s why utility companies use kWh. It maps directly to the billing rate (cost per kWh) without an intermediate conversion. At $0.15/kWh, 800 kWh = $120. The joules figure is correct but useless for budgeting.

Food energy: kcal to kJ

A meal with 650 kcal. What’s that in kilojoules?

650 kcal × 4.184 = 2,720 kJ

Most countries outside the US list food energy in kJ. American labels say “Calories” (meaning kcal). European labels say “kJ” and also “kcal” because enough people still think in calories that dropping it would cause problems. The converter handles both directions.

Natural gas: therms to kWh

A gas furnace uses 3 therms in a cold week.

3 therms × 29.3 kWh/therm = 87.9 kWh of heat energy

In BTU: 3 × 100,000 = 300,000 BTU

In joules: 3 × 105,500,000 = 316,500,000 J (316.5 MJ)

Gas billing in the US uses therms. Converting to kWh lets you compare the cost of gas heat vs. electric heat directly, which is why the conversion exists. At $1.20/therm and $0.15/kWh, that 87.9 kWh from gas cost $3.60. Getting the same heat electrically would cost $13.19. That’s why gas furnaces still exist.

Solar panel output: Wh over a day

A 400W solar panel gets 5 peak sun hours.

400W × 5h = 2,000 Wh = 2 kWh per day

In joules: 2,000 × 3,600 = 7,200,000 J

In kilocalories: 7,200,000 / 4,184 = 1,721 kcal

The kWh is the right unit here because it connects directly to what you see on your inverter display and your utility bill. The joules conversion is useful if you’re doing a physics calculation (checking motor efficiency, computing thermal loads). The kilocalorie version is useless but technically correct.

Particle physics: electronvolts to joules

A proton accelerated to 6.5 TeV (like in the LHC).

6.5 × 10¹² eV × 1.602 × 10⁻¹⁹ J/eV = 1.04 × 10⁻⁶ J = about 1 µJ

One microjoule. The same as a mosquito’s flight for 1 second.

The LHC accelerates protons to energies that, in joule terms, are genuinely tiny. The reason particle collisions matter isn’t the total energy. It’s the energy concentrated into a single subatomic particle. A proton at 6.5 TeV has the kinetic energy of a flying mosquito packed into something 10¹⁵ times smaller than the mosquito. That density is what breaks apart protons.

The electronvolt exists precisely because the joule is the wrong scale for these calculations.

Where energy unit confusion costs you

Confusing kWh and kW. A 3 kW air conditioner running for 2 hours uses 6 kWh. kW is power (rate of energy use). kWh is energy (total amount used). The “h” means time is baked in, same as the “h” in mAh. Multiplying power (kW) by time (hours) gives energy (kWh). Dividing energy (kWh) by power (kW) gives runtime (hours). This is the single most common calculation on electricity bills.

Using small-c calories for food. If someone says “that burger is 500 calories,” they mean 500 kcal = 500,000 cal = 2,092,000 joules. If you plug 500 cal (small-c) into a converter, you get 2,092 joules, which would imply a burger has less energy than a camera flash. The context always determines which calorie.

kWh vs Wh for small devices. A phone charger might draw 20W for 1.5 hours. That’s 30 Wh = 0.03 kWh. On your electricity bill, at $0.15/kWh, that’s $0.0045 per charge session. The Wh unit is the right scale here. kWh is correct but generates numbers like 0.03 that are hard to reason about.

BTU/hr vs BTU. Air conditioners are rated in BTU/hr, not BTU. A 12,000 BTU/hr air conditioner running for 3 hours uses 36,000 BTU of energy. The “/hr” is often dropped in common speech, which causes confusion when someone tries to convert an air conditioner’s “BTU rating” to kWh. The right calculation: 12,000 BTU/hr ÷ 3,412 = 3.52 kW of power draw.

Efficiency and why energy conversion matters

Most real-world energy conversions waste some input as heat.

A gas furnace rated at 95% AFUE turns 95% of the gas’s chemical energy into heat. The other 5% escapes up the flue. At 3 therms in, you get 2.85 therms of useful heat out.

An electric resistive heater is 100% efficient by definition. Every joule of electricity becomes heat. But if the electricity came from a gas turbine that’s 40% efficient, the overall efficiency from gas-in-the-ground to heat-in-the-room is 40%.

A heat pump can exceed 100% efficiency in the conventional sense. A COP (coefficient of performance) of 3.0 means 3 kWh of heat delivered per 1 kWh of electricity consumed. The extra energy comes from the outdoor air. This is why heat pumps dominate the math in cold-climate energy discussions.

The converter doesn’t calculate efficiency losses. It just does the unit math. Knowing what you’re converting to and from is the human part.

The joule in context

One joule is small. Some physical anchors to calibrate it:

The apple-lift anchor is the one physicists use. Raise a 100g apple (roughly 1 newton of force) by 1 meter against gravity: 1 joule. It’s a deliberately chosen unit size that maps to everyday mechanical work.

The fat/carbohydrate rows are useful for anyone thinking about food energy. Fat stores 9 kcal/g, carbohydrates store 4 kcal/g. In joules: 37 kJ/g vs 17 kJ/g. Fat carries more than twice the energy by weight. That’s why the body preferentially stores excess energy as fat.

The bottom line

Energy has two cross-family numbers that cover most of what you’ll ever need.

3,600,000 joules per kWh (or 3,600 joules per Wh). Comes from 1 watt × 3,600 seconds per hour.

4,184 joules per kilocalorie. Comes from the original thermodynamic definition of the calorie.

Everything else is powers of 1,000 within a family. BTU and therms are the outliers (1,055 J/BTU, 29.3 kWh/therm) and the ones you’ll look up rather than memorize.

The converter handles all of it. The real-world comparison section grounds the abstract numbers against something physical. An 800 kWh electricity bill means more once you’ve seen it sit between “lifting 1 kg to the Moon and back, 800 times” and “burning 60 gallons of gasoline.”

The numbers are the same. The unit just determines whether you see them clearly.