Energy Converter

Frequently Asked Questions

How many joules are in one calorie?

One thermochemical calorie equals exactly 4.184 joules. The "food calorie" (kcal) used on nutrition labels is 1,000 of these, or 4,184 J.

What is the difference between BTU and kWh?

A BTU is the energy needed to raise one pound of water by 1 degree F (about 1,055 J). One kWh equals 3,412 BTU, so a 1 kWh electric heater puts out roughly 3,412 BTU per hour.

Why are there so many different "calorie" definitions?

The thermochemical calorie (4.184 J), the IT calorie (4.1868 J), and the 15 C calorie (4.1855 J) all refer to the energy to heat 1 g of water by 1 C, but under slightly different conditions. For everyday use the thermochemical value is standard.

How do I convert electricity bill kWh into joules?

Multiply kWh by 3,600,000. So 500 kWh per month equals 1.8 billion joules, or 1.8 GJ.

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How This Calculator Works

Enter an energy value and choose its starting unit. The calculator converts your input to joules, the SI base energy unit, and then displays the equivalent in every other supported unit. Because every unit is defined by a fixed number of joules, a single round-trip through joules lets one input populate kilojoules, calories, kilocalories, watt-hours, kilowatt-hours, BTU, foot-pounds, electron volts, and therms at once. Results refresh the instant you type a value or change the source unit.

Formula

The conversion is a two-step ratio. First the input is normalized to joules using the source factor, then divided by the target factor: result = (value × factor_source) ÷ factor_target. The stored factors are exact or near-exact constants: 1 cal = 4.184 J, 1 Wh = 3,600 J, 1 kWh = 3,600,000 J, 1 BTU = 1,055.06 J, 1 ft·lb = 1.35582 J, 1 eV = 1.602176634 × 10⁻¹⁹ J, and 1 therm = 105,480,400 J.

Worked Example

Convert 2 kWh to other units. First normalize to joules: 2 × 3,600,000 = 7,200,000 J. To express that in kilocalories, divide by 4,184: 7,200,000 ÷ 4,184 ≈ 1,721 kcal. In BTU it is 7,200,000 ÷ 1,055.06 ≈ 6,824 BTU, and in calories 7,200,000 ÷ 4.184 ≈ 1,720,841 cal. Two kilowatt-hours is roughly a day of running a small refrigerator.

Common Mistakes

Confusing the food "Calorie" (capital C) with a calorie: the label value is a kilocalorie, so 1 food Calorie = 4,184 J, not 4.184 J.

Mixing energy (joules, kWh) with power (watts). A watt is one joule per second; energy equals power × time.

Treating BTU and therm as interchangeable: one therm equals 100,000 BTU.

Forgetting that a kilowatt-hour is 3.6 million joules, not 1,000 joules.

Tips & FAQ

Why use electron volts? At atomic scales joules give unwieldy negative exponents, so physicists use the eV (1.602176634 × 10⁻¹⁹ J) for clean numbers.

What does a kWh represent? The energy a 1,000-watt device draws over one hour, the standard unit your utility bills against.

Quick reference: a gram of fat holds ≈ 38 kJ (9 kcal), an AA battery ≈ 10 kJ, and a gallon of gasoline ≈ 125 MJ (about 35 kWh).

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