Energy Converter: kWh, Joules, BTU and Calories

Power and Energy
EssentialsHealth and FitnessUnit ConvertersMathsGeometryTimeMoneyPowerDIY

Energy Converter

Convert between joules, kilowatt hours, BTU, calories, therms and ten other energy units with exact factors. See the amount in every unit at once, what it costs at your electricity rate, and how long it runs a real appliance.

The same amount in every unittap a card to copy its value

Jump to a common conversion

Conversion tablethe row with your amount is highlighted; each first column value is a link

Every factor is exact by definition: 1 kWh is 3,600,000 J, 1 BTU is 1055.05585262 J (international table), 1 therm is 100,000 BTU, 1 kcal is 4184 J, 1 eV is 1.602176634 × 10⁻¹⁹ J. Cost and running time use the price and appliance you set above, so change them to match your own bill. The CSV, image and printed report contain the conversion you have entered, nothing else.

How to use this tool

Type an amount, pick the unit it is in and the unit you want. The answer appears in the dark panel and, underneath it, the same amount in all fifteen units at once, so a question like how many BTU, kWh and kcal are in 10 MJ is answered in one go without changing anything. Tap any card to copy its value. Swap reverses the two units, and the decimal places control applies everywhere.

The two boxes below the units turn the answer into something you can feel. Pick a currency and enter the price per kWh from your own bill, then choose an appliance or type a wattage, and the amber panel says what the energy costs and how long it would run that appliance. A 12,000 BTU amount, for example, runs a 1,500 W kettle for 2 hours 21 minutes and costs about 60 cents at 17 cents per kWh.

The input people most often get wrong is the unit itself, not the number. Nutrition labels say calories but mean kilocalories. Air conditioner labels say BTU but mean BTU per hour. Gas bills in North America say therms while the same gas elsewhere is billed in kWh or MJ. The sections below cover each of these, and the converter keeps the small calorie and the food Calorie as separate units so the factor of a thousand cannot slip past you.

Every button in the export row works on the conversion you have entered. The CSV holds your amount, the result, the factor and the full list of equivalents, the image is a shareable card of the same numbers, and Print or save as PDF produces a one page report rather than a copy of the screen. Copy link gives you an address that reopens the converter on exactly these values.

Energy and power are not the same thing

This is the mistake worth fixing before any conversion. A watt is a rate. A watt hour is an amount. Confusing them is like confusing kilometres per hour with kilometres.

A kilowatt is a thousand joules every second. Run it for an hour, which is 3,600 seconds, and you have used 3.6 million joules. That amount is what we call a kilowatt hour, and it is the unit on your electricity bill. A 1 kW heater and a 2 kW heater draw different power but can use identical energy if the smaller one runs twice as long. That is also why the appliance panel above works at all: divide energy by power and you get time.

The giveaway is the word per. Watts, horsepower and BTU per hour are all rates and belong in the power converter. Joules, kilowatt hours, calories, BTU and therms are amounts and belong here. A BTU is energy, a BTU per hour is power, and air conditioning is sold in the second while gas is billed in the first.

Which calorie do you mean

There are two, and they differ by a factor of a thousand. The small calorie is roughly the energy needed to warm a gram of water by one degree Celsius, fixed at exactly 4.184 joules in its thermochemical form. The kilocalorie is a thousand of those.

Every number on a nutrition label is kilocalories, usually written with a capital C as Calorie or simply mislabelled as calories. A 2,000 Calorie daily intake is 2,000 kilocalories, which is 8.37 megajoules, or about 2.3 kilowatt hours. That is roughly what a fridge uses in a day, which is a useful sense of scale for how little energy a human actually runs on. Labels in Europe and Australia print kilojoules alongside, and the same 2,000 kcal appears there as 8,368 kJ.

Which BTU, and what a therm and an MMBtu are

There are several BTU definitions because the unit was originally tied to the temperature of a pound of water, and water does not warm evenly. The international table BTU, exactly 1055.05585262 J, is the one used in engineering, in gas billing and in this converter. The thermochemical BTU is 1054.35 J, and the 59 °F BTU is 1054.80 J. The differences sit in the fourth significant figure, which is invisible on a bill and can matter in a contract.

A therm is exactly 100,000 BTU, so 105.506 MJ or 29.3071 kWh, and it is the billing unit for natural gas across most of the United States and in the United Kingdom. An MMBtu is a million BTU, ten therms, and it is the trading unit for gas at the wholesale level; the double M comes from the Roman numeral for a thousand written twice. A dekatherm is the same thing under another name. One US customary quirk: some American gas utilities define the therm from the 59 °F BTU, which makes their therm 105.480 MJ, about 0.02 percent smaller. The converter uses the international table version.

A worked check you can repeat on paper

Start with one kilowatt hour. A kilowatt is 1,000 joules per second and an hour is 3,600 seconds, so multiply and you get 3,600,000 joules, or 3.6 MJ. Nothing is rounded, because both the watt and the second are defined exactly.

Now into BTU. One BTU is 1055.05585262 joules, so divide 3,600,000 by that and you get 3,412.14 BTU. That figure is worth remembering, because it is what converts a heating output in BTU per hour into kilowatts and back. Run it the other way for an air conditioner: 12,000 BTU divided by 3,412.14 is 3.517 kWh, and since the label really means 12,000 BTU per hour, the unit delivers 3.517 kW of cooling.

And into food Calories: 3,600,000 divided by 4,184 is 860.42 kcal. So a single kilowatt hour of electricity carries about the same energy as a large meal, which says more about how much energy food holds than about how little electricity costs. The converter reports exactly these three figures for its default input of 1 kWh, so if yours differ, check which calorie or which BTU you are holding.

Logarithmic scale from 1 joule to 100 gigajoules with unit definitions marked above the axis, BTU, food Calorie, kilowatt hour, therm, million BTU and tonne of TNT, and everyday amounts below it, from lifting an apple to a year of household electricity
Notice how far apart the units sit. A BTU and a kilowatt hour differ by more than three decades, and a home uses about nine tonnes of TNT worth of electricity in a year without anyone noticing.

What the cost and running time actually mean

The cost line is the energy converted to kilowatt hours and multiplied by your price per kWh. It is the energy charge only. Real bills add a fixed daily or monthly charge, sometimes tiered pricing, and taxes, so treat the figure as the marginal cost of using that energy rather than what the bill will say. Residential rates vary a lot: recent US state averages run from about 11 cents per kWh to more than 40 in Hawaii, and UK household rates have sat in the mid twenties of pence, which is why each currency in the list comes with its own placeholder.

The running time is the energy divided by the appliance power. The wattages in the list are typical running figures, not nameplate maximums: a fridge is rated at several hundred watts but averages about 100 because the compressor cycles on and off, and a kettle draws its full 1,500 W the whole time it is on. When you know the real figure for your own appliance, choose Custom wattage and type it. For hours worked out from a battery rather than the grid, the Battery Capacity Calculator handles the voltage and capacity side.

Mistakes that catch people out

Writing kW when you mean kWh. A battery holding 5 kWh and a charger delivering 5 kW are different claims. The first is a size, the second is a speed. Divide one by the other and you get the charging time, about an hour, which is the only sensible thing to do with the pair.

Mixing up calories and Calories. A factor of a thousand separates them. If a figure came from a food label it is kilocalories, whatever the label calls them.

Treating an air conditioner rating as energy. 12,000 BTU on the box means 12,000 BTU per hour. Converting it here gives 3.517 kWh, which is the energy the unit removes from the room in one hour of full running, not the electricity it uses. A unit with an EER of 12 draws about a third of that from the wall.

Assuming there is only one BTU. There are several definitions differing in the fourth significant figure. This tool uses the international table BTU, which is the one used in nearly all engineering work.

Treating a therm as a metric unit. It is not. A therm is exactly 100,000 BTU, and it turns up on gas bills in North America and the United Kingdom while the rest of the metric world bills the same gas in kilowatt hours or megajoules.

Reading a bill in cubic feet or cubic metres as energy. Gas meters measure volume. The utility multiplies by a calorific value and a pressure factor to get therms or kWh, and that factor changes slightly from month to month. Convert the energy figure on the bill, never the volume.

Popular energy conversions

The conversions people look up most, grouped by what they are usually for. Each link opens the converter ready to go with its own address, so you can bookmark or share it.

Electricity: kWh, MJ and BTU

1 kWh to MJ2 kWh to MJ3 kWh to MJ5 kWh to MJ10 kWh to MJ15 kWh to MJ20 kWh to MJ25 kWh to MJ30 kWh to MJ50 kWh to MJ100 kWh to MJ200 kWh to MJ500 kWh to MJ1000 kWh to MJ1 MJ to kWh2 MJ to kWh5 MJ to kWh10 MJ to kWh20 MJ to kWh25 MJ to kWh50 MJ to kWh100 MJ to kWh200 MJ to kWh500 MJ to kWh1000 MJ to kWh1 kWh to BTU2 kWh to BTU3 kWh to BTU4 kWh to BTU5 kWh to BTU10 kWh to BTU20 kWh to BTU50 kWh to BTU100 kWh to BTU500 kWh to BTU1000 kWh to BTU1500 kWh to BTU2000 kWh to BTU1 kWh to J2 kWh to J5 kWh to J10 kWh to J1000 J to kWh10000 J to kWh100000 J to kWh1000000 J to kWh3600000 J to kWh1 GJ to kWh2 GJ to kWh5 GJ to kWh10 GJ to kWh100 GJ to kWh

Heating and cooling: BTU to kWh by air conditioner size

Air conditioner and heat pump capacities. Remember the label means BTU per hour, so the kWh figure is the cooling delivered in one hour.

1000 BTU to kWh5000 BTU to kWh6000 BTU to kWh8000 BTU to kWh9000 BTU to kWh10000 BTU to kWh12000 BTU to kWh14000 BTU to kWh15000 BTU to kWh18000 BTU to kWh24000 BTU to kWh30000 BTU to kWh36000 BTU to kWh48000 BTU to kWh60000 BTU to kWh100000 BTU to kWh1 BTU to kJ10 BTU to kJ100 BTU to kJ1000 BTU to kJ5000 BTU to kJ10000 BTU to kJ1 BTU to J10 BTU to J100 BTU to J

Gas bills: therms and MMBtu

1 therm to kWh2 therm to kWh5 therm to kWh10 therm to kWh20 therm to kWh25 therm to kWh30 therm to kWh40 therm to kWh50 therm to kWh75 therm to kWh100 therm to kWh150 therm to kWh200 therm to kWh300 therm to kWh500 therm to kWh1 therm to MJ10 therm to MJ1 MMBtu to kWh2 MMBtu to kWh5 MMBtu to kWh10 MMBtu to kWh20 MMBtu to kWh50 MMBtu to kWh100 MMBtu to kWh1000 MMBtu to kWh1 MMBtu to therm1 MMBtu to GJ10 MMBtu to GJ100 kWh to therm500 kWh to therm1000 kWh to therm

Food labels: kcal and kJ

50 kcal to kJ100 kcal to kJ150 kcal to kJ200 kcal to kJ250 kcal to kJ300 kcal to kJ400 kcal to kJ500 kcal to kJ600 kcal to kJ700 kcal to kJ800 kcal to kJ1000 kcal to kJ1200 kcal to kJ1500 kcal to kJ1800 kcal to kJ2000 kcal to kJ2500 kcal to kJ3000 kcal to kJ100 kJ to kcal250 kJ to kcal500 kJ to kcal1000 kJ to kcal1500 kJ to kcal2000 kJ to kcal2500 kJ to kcal3000 kJ to kcal4184 kJ to kcal5000 kJ to kcal8000 kJ to kcal10000 kJ to kcal2000 kcal to kWh2500 kcal to kWh1 kWh to kcal2 kWh to kcal5 kWh to kcal10 kWh to kcal1 cal to J10 cal to J100 cal to J1000 cal to J

Small amounts: joules, watt hours and electronvolts

100 J to kJ500 J to kJ1000 J to kJ5000 J to kJ10000 J to kJ1 Wh to J10 Wh to J100 Wh to J500 Wh to J1000 Wh to J1 eV to J

How this was checked

Every factor was checked against its definition rather than a table. The kilowatt hour came out at exactly 3,600,000 J from 1,000 watts times 3,600 seconds, the international table BTU at 1055.05585262 J, the therm at exactly 100,000 BTU, the MMBtu at exactly a million, the thermochemical calorie at 4.184 J and the food Calorie at 4,184 J. The electronvolt uses the exact 2019 SI value, and a tonne of TNT is exact by convention at 4.184 GJ.

Round tripping was run across every pair of the fifteen units, all 225 combinations, converting a value out and back and checking it returned to within one part in a trillion. There were no failures. The derived figures quoted above were checked the same way: a kilowatt hour is 3,412.14 BTU and 860.42 kcal, a therm is 29.3071 kWh, and 12,000 BTU is 3.5169 kWh.

The exports were tested by entering 12,000 BTU to kWh, downloading each one and reading it back. The CSV, the image and the printed report each carried that conversion and its equivalents, and nothing from the default state.

Common questions

How do you convert kWh to joules?

Multiply by 3,600,000. A kilowatt is 1,000 joules per second and an hour is 3,600 seconds, so 1 kWh is exactly 3.6 million joules, or 3.6 MJ. Ten kWh is 36 MJ. To go the other way, divide joules by 3,600,000.

How many kWh is 12,000 BTU?

12,000 BTU is 3.5169 kWh, because one BTU is 1055.05585262 joules and one kWh is 3,600,000 joules. Note that an air conditioner rated at 12,000 BTU means 12,000 BTU per hour, which is a power of 3.517 kW of cooling, not an amount of energy.

How many kWh are in a therm of gas?

One therm is 29.3071 kWh. A therm is exactly 100,000 BTU, and 100,000 times 1055.05585262 joules divided by 3,600,000 gives 29.3071. A million BTU (MMBtu) is ten therms, or 293.071 kWh.

What is the difference between a calorie and a Calorie?

A factor of a thousand. The small calorie, written cal, is 4.184 joules. The food Calorie on a nutrition label, written with a capital C or as kcal, is 4,184 joules. A 2,000 Calorie diet is 2,000 kcal, which is 8.368 MJ or 2.32 kWh.

Is a BTU energy or power?

A BTU is energy, the same kind of quantity as a joule or a kilowatt hour. BTU per hour is power, the same kind of quantity as a watt. Furnaces, boilers and air conditioners are rated in BTU per hour even though the label often just says BTU, and gas is billed in therms, which are 100,000 BTU each.

Does the cost shown match my electricity bill?

Only if you enter your own rate. The default price is a placeholder for a typical residential rate, and real bills add fixed charges, tiered pricing and taxes on top of the energy charge. Set the price per kWh from your latest bill and the cost line will match the energy portion of it.

Related Calculators and Reference