Energy Converter

How do you convert energy units?

To convert, multiply by the starting unit’s value in joules, then divide by the target unit’s value in joules. All energy units measure the same thing, the capacity to do work or produce heat, and each has a fixed value in joules. One joule is the work done by a force of one newton moving an object one meter.

A worked example

To convert 250 kcal to kilojoules: 250 × 4,184 = 1,046,000 J, then 1,046,000 ÷ 1,000 = 1,046 kJ. The single shortcut is 4.184, so multiply kilocalories by 4.184 to get kilojoules, or divide kilojoules by 4.184 to get kilocalories with the kJ to kcal converter.

Calories, kilocalories and the food Calorie

The word calorie causes more confusion than any other energy unit. A small calorie (cal) is the energy that warms one gram of water by about one degree Celsius, and this converter uses the thermochemical definition of exactly 4.184 J. The Calorie on a food label is a kilocalorie, 1,000 small calories or 4,184 J. So a snack labeled 100 Calories contains 100 kcal, which is 418.4 kJ, not 100 cal. If you see a value with a capital C or in the Nutrition Facts panel, treat it as kcal. For small calories and joules, the joules to calories converter shows the 4.184 factor at work.

A second definition, the International Table calorie, equals 4.1868 J. It differs from the thermochemical calorie by less than 0.07%, which matters only for precise engineering data.

What is the SI unit of energy, and how is the joule defined?

The joule (J) is the SI unit of energy: the work done when a force of one newton moves something one meter, so 1 J = 1 N·m = 1 kg·m²/s². It is also one watt for one second (1 J = 1 W·s). For a feel of the size, lifting a 102 g apple one meter against standard gravity takes about one joule; the force converter covers the newton itself.

The BIPM SI Brochure lists the joule as a coherent derived unit and accepts one more energy unit for use with the SI, the electronvolt (eV), which has been exactly 1.602176634 × 10⁻¹⁹ J since the 2019 redefinition. Everything else on this page, including the kilowatt-hour, the calorie, the BTU and the foot-pound, is a non-SI unit with an exact value in joules, as tabulated in NIST SP 811. ISO 80000-5 sets the names and symbols for heat and other thermodynamic quantities.

Joules versus newton-meters

Torque is also a force times a distance, so it shares the base units kg·m²/s² with energy. The SI Brochure recommends writing torque in newton-meters (N·m) and energy in joules so the two are never confused: a bolt tightened to 50 N·m has not stored 50 J. Foot-pounds follow the same split, with foot-pound-force for energy and pound-feet for torque.

Which energy units are used where?

Kilocalories and kilojoules describe food, kilowatt-hours describe electricity, BTU and therms describe heating and cooling in the US, and joules are the scientific standard everywhere.

• Kilocalories (Calories): food labels and diet tracking in the United States, where the Nutrition Facts panel lists Calories, and alongside kilojoules on labels in the European Union.
• Kilojoules: the main food energy unit on labels in Australia and New Zealand, and required next to kcal on EU packaging. Nutrition science and sports physiology also increasingly use kJ.
• Kilowatt-hours: electricity bills almost everywhere, electric vehicle battery capacity and home solar output. One kWh is the energy used by a 1,000-watt appliance running for one hour; the kWh to BTU converter helps compare electric and gas heating.
• BTU: heating, ventilation and air conditioning in the United States, furnace and water heater ratings, and natural gas, which US utilities often bill in therms of 100,000 BTU.
• Joules and megajoules: physics, chemistry and engineering, fuel energy content (often in MJ per kilogram or per liter) and battery specifications.
• Foot-pounds: US mechanical engineering and firearm muzzle energy. Note that torque is also written in pound-feet, but torque is a different quantity; use the torque converter for that.

How do kWh, BTU and therms compare for home heating?

One kilowatt-hour equals about 3,412.14 BTU, and one therm, 100,000 BTU, equals about 29.3 kWh, so electric and gas heating can be compared once both are in the same unit. The kWh to BTU converter does the arithmetic; US gas bills use therms, while many other countries bill gas in kWh.

• Heating and cooling ratings are power, not energy. A furnace rated at 80,000 BTU per hour of input burns fuel at about 23.45 kW, and a 12,000 BTU/h air conditioner, one “ton” of cooling, moves about 3.517 kW of heat. The power converter handles BTU/h, watts and horsepower.
• Heat pumps multiply electricity. The coefficient of performance (COP) is heat delivered divided by electricity used, so a heat pump running at a COP of 3 turns 1 kWh of electricity into 3 kWh, about 10,236 BTU, of heat.
• Furnaces lose some heat up the flue. In the US, gas furnaces carry an AFUE (Annual Fuel Utilization Efficiency) rating; at 95% AFUE, one therm delivers about 27.8 kWh of heat to the home.

To compare running costs, convert each fuel’s price to a price per kWh of delivered heat, allowing for COP or efficiency. Thermostat settings in Fahrenheit or Celsius are covered by the temperature converter.

What is the difference between energy and power?

Energy is an amount; power is the rate at which that energy is used or delivered. A watt is one joule per second, so a kilowatt-hour is simply 1,000 joules per second sustained for 3,600 seconds, which is 3.6 million joules. A 2 kW kettle running for 3 minutes uses 0.1 kWh, the same energy as a 10 W LED bulb left on for 10 hours. When a device is rated in watts or horsepower, that is power, and the power converter handles those units. When a bill or battery is rated in kWh or joules, that is energy.