Energy Unit Converter
Online energy unit converter - convert between joules, kilowatt-hours, calories, kilocalories, BTU, electron-volts and more
What is Energy Unit Conversion?
This energy unit converter lets you quickly convert between different energy units. It supports 13 common energy units, including SI units (joules, kilojoules, megajoules, gigajoules), electrical units (watt-hours, kilowatt-hours), thermal units (calories, kilocalories), imperial units (BTU, foot-pounds), and scientific units (electron-volts, ergs). Whether you're calculating electricity costs, converting food calories, or working on physics problems, this tool gives you accurate results instantly. All calculations run locally in your browser - no data is uploaded. Energy units connect everyday use, physics, nutrition, utilities, and engineering. Joules, kilojoules, calories, kilocalories, watt-hours, kilowatt-hours, and BTU appear in different contexts, so conversion helps compare batteries, electricity consumption, heating output, food energy, lab values, and technical specifications. For contracts, invoices, or measurement reports, it is important to distinguish energy from power and time-based consumption, because watts and watt-hours are often confused even though they describe different things.
How to Use
How to use
- Enter the value you want to convert
- Select the source unit (from) and target unit (to)
- Click the Convert button to see the result
- Check Related Conversions for other unit combinations
Context Matters
- Energy units often come from different fields: joules for physics, calories for nutrition, kWh for electricity, and BTU for HVAC.
- Do not mix energy with power; watts and kilowatts describe rate, while joules and kWh describe total energy.
Use Cases
Technical Principle
The joule (J) is the SI derived unit for energy, work, and heat, defined since the 2019 SI redefinition as kg·m^2·s^-2 with the second anchored to the caesium-133 hyperfine transition (9 192 631 770 Hz) and the kilogram fixed by the Planck constant h = 6.626 070 15 × 10^-34 J·s. Every other unit in the converter is expressed as a fixed scalar multiple of the joule, so conversion is a single floating-point multiplication factor[from] / factor[to] applied to the input value, where each factor stores the unit's joule equivalent. The core constants are: 1 cal_th = 4.184 J exactly (thermochemical calorie); 1 cal_IT = 4.1868 J (International Table calorie used by ISO 31-4); 1 kcal = 1000 cal (the food-label Calorie); 1 Wh = 3600 J because W·s = J and 1 h = 3600 s; 1 kWh = 3 600 000 J = 3.6 MJ; 1 BTU_IT = 1055.056 J (1 therm = 100 000 BTU ≈ 105.5 MJ); 1 ft·lbf = 1.355 817 948 331 4004 J (using g_n = 9.806 65 m/s^2); 1 erg = 10^-7 J (CGS); 1 eV = 1.602 176 634 × 10^-19 J exactly per the 2019 SI fixed value of the elementary charge; 1 toe (tonne of oil equivalent) = 41.868 GJ by IEA convention. Power is the time derivative of energy (1 W = 1 J/s), so a 100 W bulb running 1 h consumes 100 W × 3600 s = 360 000 J = 0.1 kWh - mixing watts (rate) with watt-hours (total energy) is the most common unit-confusion bug in this domain. Numerical accuracy is bounded by IEEE 754 double precision (about 15-17 significant decimal digits, machine epsilon 2^-52 ≈ 2.22 × 10^-16). Converting between values that differ by very large ratios (1 eV to 1 GJ spans 28 orders of magnitude) is exact in the conversion factor itself but loses precision in the input mantissa; for extreme magnitudes the chained operation a × (f1 / f2) is preferred over (a × f1) / f2 because the intermediate a × f1 can overflow to Infinity or underflow to 0 when the input is near the representable limit (≈ 1.8 × 10^308 / ≈ 5 × 10^-324).
- Joule definition: 1 J = 1 kg·m^2·s^-2; SI base since 2019 fixes h = 6.626 070 15 × 10^-34 J·s exactly.
- Calorie ambiguity: thermochemical calorie cal_th = 4.184 J exactly; IT calorie cal_IT = 4.1868 J; food label Calorie (capital C) = 1 kcal = 4184 J - mixing them up shifts results by 0.07%.
- Electrical: 1 Wh = 3600 J, 1 kWh = 3.6 MJ; watts measure rate (J/s), watt-hours measure total energy, so a 60 W bulb for 10 h = 0.6 kWh ≠ 60 W.
- Imperial/HVAC: 1 BTU_IT = 1055.056 J, 1 therm = 100 000 BTU ≈ 105.506 MJ ≈ 29.3 kWh; 1 ft·lbf = 1.355 817 948 331 4 J under standard gravity g_n = 9.806 65 m/s^2.
- Physics: 1 eV = 1.602 176 634 × 10^-19 J exactly (CODATA 2019 fixed); 1 erg = 10^-7 J; 1 toe = 41.868 GJ (IEA).
- IEEE 754 double precision gives ~15-17 significant decimal digits; values spanning 28 orders of magnitude (eV ↔ GJ) round to representable mantissa width and should display only ~6-9 significant figures to avoid suggesting false precision.
- Chain conversions as value × (f_from / f_to) rather than (value × f_from) / f_to to avoid intermediate overflow to Infinity (above ~1.8 × 10^308) or underflow to 0 (below ~5 × 10^-324).
Examples
Kilowatt-hours to joules and kilocalories
1 kWh = 3,600,000 J = 3.6 MJ ≈ 860 kcalFood-label calories to joules
1 kcal = 4,184 J; a 30-minute run burning 300 kcal ≈ 1.26 MJ ≈ 0.35 kWhGasoline energy content (everyday reference)
1 US gallon of gasoline ≈ 33.7 kWh ≈ 121.3 MJ ≈ 28,980 kcalHousehold appliance energy use
A 1.5 hp (≈1,100 W) air conditioner running 1 hour ≈ 1.1 kWh ≈ 3.96 MJ; a 60 W bulb for 10 hours = 0.6 kWhBTU and therm (HVAC and utility billing)
1 BTU = 1,055.06 J; 1 therm = 100,000 BTU ≈ 105.5 MJ ≈ 29.3 kWhPhysics scale (SI to electron-volts)
1 eV = 1.602 × 10⁻¹⁹ J; 1 J ≈ 6.242 × 10¹⁸ eVFAQ
Which energy units are supported?
Joule (J) and its multiples kJ/MJ/GJ, calorie (cal), kilocalorie (kcal, the food calorie), watt-hour (Wh), kilowatt-hour (kWh), British thermal unit (BTU), foot-pound (ft·lb), electron-volt (eV), and ton of TNT equivalent. Pick any pair and convert in real time.
Is 'calorie' the same as 'kilocalorie'?
On nutrition labels, 'Calorie' (capital C) usually means kilocalorie - 1 food Calorie = 1000 thermochemical calories = 4184 J. In physics and chemistry, 'calorie' (lowercase c) means the small calorie. The page treats them as separate units to avoid the famous confusion.
How do I convert kWh to joules?
1 kWh = 3,600,000 J = 3.6 MJ. A typical home appliance running at 1 kW for an hour consumes 3.6 MJ. The page does this conversion in real time.
What's the relationship between energy and power?
Energy (joules) = power (watts) × time (seconds). 1 watt sustained for 1 second = 1 joule. The page converts energy units; for power conversions (W, hp, BTU/h) use a power converter.
Why is the food calorie 4184 J and not 4186 J?
Several 'calorie' definitions exist with slightly different values: thermochemical (4.184 J), 15 °C (4.1855 J), International Table (4.1868 J). Modern nutrition labels in the EU use International Table (4.184 kJ per Cal); US labels often use thermochemical. The page uses the thermochemical value by default.
What's a ton of TNT equivalent?
1 ton TNT = 4.184 GJ (defined by convention). It is used to express explosive yield: the Hiroshima bomb was about 15 kilotons TNT = 6.3 × 10^13 J. Useful for pop-science comparisons; not used in engineering.
Are conversions exact?
Most are exact by definition (J, eV, ton TNT, kWh). The calorie has multiple definitions, so cal-based conversions depend on which one you pick - the page lists thermochemical and IT values separately if you need a specific match.