Energy Unit Converter

1 kWh is 3,412.141633 Btu, or 859.85 kcal on the International Table scale

One kilowatt-hour is the energy of 1 kW of power used for one hour: 3.6 MJ, or 3,600,000 J. Divide by 1,055.05585262 J per International Table Btu and you get 3,412.141633 Btu. In kilocalories the answer depends on which calorie you use: 859.85 kcal with the International Table calorie (1 cal = 4.1868 J) and 860.42 kcal with the thermochemical calorie (1 cal = 4.184 J), the one most often used to convert kcal to kJ.

The converter above turns your input into joules first and then divides by the target unit, so every result comes from the same exact factors. Both calories are listed separately, and picking one also shows the other. Values given as power, such as kW, horsepower or Btu/h, go through the "Power × time" mode, which multiplies by the hours of use to get energy. The "Energy scale comparison" bar places the result on a log scale and counts how many laptop charges, bananas or days of food energy it equals.

When you need to convert energy units

• Comparing electricity and heat: electricity is sold in kWh, while heating fuel is often quoted in Btu or therms (about 100,000 Btu). Putting both in one unit lets you compare what each costs for the same amount of heat.
• Reading food labels: labels in the United States show Calories, which are kilocalories, while EU labels give energy in both kJ and kcal. Divide kJ by 4.184 to get kcal.
• Sizing heating and cooling equipment: air conditioners and furnaces are rated in Btu/h. That is a rate of heat, a power, so you multiply by the hours of use to get energy.
• Reading energy statistics: international statistics often use the tonne of oil equivalent (toe), defined as 41.868 GJ, or the tonne of coal equivalent (tce), 29.3076 GJ.
• Physics and chemistry problems: atomic and particle physics use the electronvolt (eV), and older CGS texts use the erg.

How the conversion works

1. Convert the input to joules: input × joules per unit. 1 kWh is 1 × 3,600,000 = 3,600,000 J.
2. Divide by the target unit: joules ÷ joules per target unit. 3,600,000 J ÷ 4,186.8 J = 859.8452279 kcal(IT).
3. Power × time: watts × hours × 3,600 = joules. A 2 kW heater running for 3 hours uses 2,000 × 3 × 3,600 = 21,600,000 J = 6 kWh.
4. Display: results show 10 significant digits, and values of 10¹⁵ or more, or below 10⁻⁶, switch to exponent notation.

Factors follow the defined values in NIST SP 811, Appendix B.9, and the EIA unit table, checked on 2026-10-12. 1 eV = 1.602176634 × 10⁻¹⁹ J is an exact value since the 2019 SI revision.

Why there are two calories

The calorie began as the heat needed to raise 1 g of water by 1 °C. Today it is fixed in joules, and more than one definition is in use. The two common ones are the thermochemical calorie, cal(th) = 4.184 J, and the International Table calorie, cal(IT) = 4.1868 J. NIST lists both as exact values. They differ by about 0.07%, which rarely matters in daily life but does when you need an exact figure to match a report or a textbook answer.

The "calorie" on a food label is really the kilocalorie. Because both definitions are in use, a food energy converted to kJ can differ slightly between products, depending on whether 4.184 or 4.1868 was applied.

kW, horsepower and Btu/h are power: multiply by time

A watt is one joule per second, so kW or horsepower cannot be turned into kcal or kWh on its own. Multiply the power by the time to get energy. The table below uses the rated figures as given. Real consumption depends on whether the device actually runs at that output the whole time.

The Btu/h or kW on an air conditioner is its cooling capacity, the heat it removes from a room, and not the electricity it draws. A furnace's output rating is likewise the heat it delivers, not the fuel it burns. To estimate a power bill or fuel cost, use the electrical input power or the fuel consumption from the product's own specification.

Fuel energy content: gross and net values

Every fuel has a gross heating value, which includes the latent heat of the water vapor produced when it burns, and a lower net value, which leaves that heat out. The gap can be several percent. Each country publishes its own reference table, and the tables are revised over time, so two sources can give different numbers for the same fuel.

This page does not list per-fuel values because the one official table checked, the EIA's sample Btu factors, gives no year and does not say whether its figures are gross or net. For a report or a calculation that needs to be exact, take the value from the current table of your own country's energy agency and note whether it is gross or net.

Reference points for the scale comparison

These are the reference points used by the "Energy scale comparison" above, with their sources. The tool divides your value by each one to say, for example, "12 bananas". Charging losses, combustion efficiency and digestion are not counted; the values are nominal.

Common mistakes

• Treating kW as kWh: kW is power and kWh is energy. A 2 kW heater uses 2 kWh only if it runs flat out for exactly one hour.
• Mixing 859.85 and 860.42: if the calorie definition changes partway through a calculation, the last digits will not match. Use the International Table calorie to match toe-based statistics and the thermochemical calorie to match kcal-to-kJ tables.
• Reading air-conditioner Btu/h as power draw: 12,000 Btu/h (3.517 kW) is cooling capacity. The electrical input is a different figure listed in the specifications.
• Ignoring the kind of horsepower: metric horsepower (PS) is 735.49875 W and mechanical horsepower (hp) is 745.6998716 W, about 1.4% apart.
• Mixing the EC and U.S. therm: the two differ by about 0.02%. For exact U.S. gas billing figures use the U.S. therm, 105.4804 MJ.
• Comparing gross and net heating values: a fuel's net value is lower than its gross value, so always state which one you used in an efficiency figure.

Limits and cautions

• The tool does the mathematical conversion between units only. It does not include device efficiency, charging losses, combustion efficiency or how much of a food's energy the body absorbs.
• Only two calorie definitions are covered: thermochemical (4.184 J) and International Table (4.1868 J). Others, such as the 15 °C calorie, are not included.
• The therm is the EC definition (105.506 MJ), which differs from the U.S. therm by about 0.02%.
• Reference-point values are nominal figures from their sources. Real fuels and foods vary with composition and measurement conditions.
• Negative energy is not handled. Results are rounded to 10 significant digits.
• The scale comparison covers 1 kJ to 100 GJ. Smaller or larger values sit at the end of the scale, while the reference counts are still calculated.

Sources

• 01NIST SP 811 Appendix B.9 — Factors for units listed by kind of quantitycal_th 4.184 J and cal_IT 4.1868 J (exact), kWh 3.6 MJ, therm (EC) 1.05506 × 10⁸ J, therm (U.S.) 1.054804 × 10⁸ J, ton of TNT 4.184 × 10⁹ J, Btu_IT/lb 2.326 kJ/kg, horsepower and Btu/h factors
• 02NIST CODATA 2022 — electron volt-joule relationship1 eV = 1.602176634 × 10⁻¹⁹ J (exact)
• 03U.S. EIA — International Energy Statistics: Units1 TOE = 41.868 GJ, 1 TCE = 29.3076 GJ = 0.7 TOE — checked 2026-10-12
• 04USDA FoodData Central — Bananas, raw (173944)89 kcal per 100 g, 105 kcal for one medium banana (118 g)
• 05Apple — MacBook Air technical specifications13-inch (M5): 53.8 Wh lithium-polymer battery — checked 2026-10-12
• 06U.S. FDA — Calories on the Nutrition Facts Label"2,000 calories a day is used as a general guide for nutrition advice" — checked 2026-10-12

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