Unit Converters guide
Calories, Kilojoules, kWh and BTU: 6 Real Uses for Energy Units
· 3 min read
Energy units all measure the same thing, the capacity to do work, so they convert with fixed factors. The ones you'll need most: 1 kcal (a food Calorie) = 4.184 kJ, 1 kWh = 3.6 MJ = 3,412 BTU, and 1 Wh = 3,600 J. Food labels, power bills, heating systems and batteries simply grew up with different units.
Here are six everyday places where energy conversions come up, with the numbers worked out.
The core conversion factors
| Unit | In joules |
|---|---|
| Calorie, small (cal) | 4.184 |
| Kilocalorie (kcal) | 4,184 |
| Watt-hour (Wh) | 3,600 |
| Kilowatt-hour (kWh) | 3,600,000 |
| British thermal unit (BTU) | ≈ 1,055.06 |
| Foot-pound (ft·lbf) | ≈ 1.356 |
| Electronvolt (eV) | ≈ 1.602 × 10⁻¹⁹ |
1. Reading food labels
Nutrition labels in the US list "Calories" with a capital C, which are kilocalories. Labels in Europe and Australia often list kilojoules first.
- A 250 kcal snack is 1,046 kJ.
- An 840 kJ portion is about 200.8 kcal.
- A 2,000 kcal daily reference intake is 8,368 kJ.
A quick estimate: kJ ÷ 4 is a little over the kcal figure; kcal × 4.2 is close to kJ.
Watch out: the small calorie (cal) is a thousand times smaller than a food Calorie. In the converter, choose Kilocalorie (kcal) for anything from a nutrition label, not "Calorie (cal)". To estimate how many kilocalories you burn a day, the BMR Calculator gives a starting figure.
2. Understanding your electricity bill
Electricity is billed in kilowatt-hours: the energy used by a 1-kilowatt appliance running for an hour. A 2 kW heater left on for 3 hours uses 6 kWh.
In other units, 1 kWh is:
- 3,600 kJ (3.6 MJ)
- 860.4 kcal
- 3,412.1 BTU
So 3 kWh, a modest amount of household use, holds as much energy as about 2,581 kcal, more than a day's food for many adults.
3. Sizing an air conditioner or heater
Air conditioners in the US are rated in BTU per hour. That is a power rating, but converting the energy unit gives you the electrical-style figure: 12,000 BTU = 3.517 kWh, so a "12,000 BTU" unit delivers about 3.5 kW of cooling. (Its electricity use is lower, since air conditioners move more heat than they consume.)
4. Comparing batteries
Laptop and power-bank batteries are often rated in watt-hours. A 50 Wh battery stores 180 kJ. For comparison, 1 kcal is about 1.16 Wh, so a 50 Wh battery holds roughly as much energy as 43 food Calories. It puts into perspective how energy-dense food is.
5. Physics and chemistry homework
Atomic and particle physics use the electronvolt, the energy an electron gains crossing one volt: 1 eV ≈ 1.602 × 10⁻¹⁹ J. The converter shows very small results like this in scientific notation, such as 1.60217663e-19.
6. Mechanical work in imperial units
Engineers working in imperial units use the foot-pound (ft·lbf), the work done lifting one pound by one foot. 1 ft·lbf ≈ 1.356 J, which shows up in torque wrench and impact-energy specs.
How to use the Energy Converter
The Energy Converter covers all nine units above:
- Enter a number in Value.
- Pick the From and To units: joule, kilojoule, calorie, kilocalorie, watt-hour, kilowatt-hour, British thermal unit, electronvolt or foot-pound. The ⇄ button swaps them.
- Read the result, or look at the table to see the same energy in all nine units.
Results show up to 10 significant digits, and very large or very small values use scientific notation.
Energy vs power: a common mix-up
Energy is an amount (joules, kWh, kcal). Power is a rate of energy use (watts, kW). A 60 W bulb uses 60 joules every second; leave it on for 10 hours and it has used 600 Wh, or 0.6 kWh. The converter works with energy amounts, so convert power ratings only after multiplying by time.
FAQ
Why are there different BTU values online?
There are several historical definitions of the BTU. This converter uses the International Table BTU, about 1,055.06 J. The others differ by a fraction of a percent.
How many joules are in a food Calorie?
4,184 J, because a food Calorie is a kilocalorie.
Can I convert kWh to liters of fuel?
Not with a unit conversion alone. That depends on the fuel's energy density, which differs between gasoline, diesel and others.
Turn calories into kilojoules, kWh into BTU and more with the Energy Converter.