Energy Unit Converter
Convert between any pair of energy units — joule, kilojoule, watt-hour, kilowatt-hour, calorie, kilocalorie, BTU, therm, electronvolt and more — using factors verified against NIST SP 811 and the BIPM SI Brochure. Type a value and the result updates instantly.
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What is energy, and why so many units?
Energy is the capacity to do work — the universal currency of physics that flows between kinetic, potential, thermal, chemical, electrical, and radiant forms. In SI units, one joule (J) is the work done when a force of one newton acts through one meter, equivalent to one watt of power dissipated for one second. Real-world energy values, however, span more than forty orders of magnitude — from a single photon at 10⁻¹⁹ J to a national power grid's annual output near 10¹⁸ J — so each industry inherited a unit that keeps the numbers manageable in its niche.
Heat engineers grew up with calories and BTU. Electricity meters tick in kilowatt-hours. Nutrition labels print kilocalories (sometimes labelled "calories"). Particle physicists count electronvolts. Mechanical engineers in the United States quote foot-pounds. Astrophysicists still use ergs. Every one of these is just an alternate name for a number of joules — and this page shows the exact conversion factors and lets you move between them in real time.
The energy units, explained
Joule (J), kilojoule (kJ), megajoule (MJ) — the SI baseline
The joule is the coherent SI derived unit: 1 J = 1 N·m = 1 W·s. It is small for most engineering work — a single beat of your heart pumps about 1 J — so multiples are common: 1 kJ = 10³ J, 1 MJ = 10⁶ J. Food labels in the EU are required to show energy in kJ alongside kcal; a typical breakfast cereal serving is 700–900 kJ. A megajoule is roughly the energy in 23 grams of gasoline.
Watt-hour (Wh), kilowatt-hour (kWh), megawatt-hour (MWh) — the electricity unit
1 Wh = 3,600 J because power × time = energy: 1 watt for 3,600 seconds equals 3,600 joules. The kilowatt-hour (kWh = 3.6 MJ = 3,600,000 J) is the unit your electricity utility bills you in. A modern fridge consumes 300–500 kWh per year; a U.S. household averages around 10,500 kWh annually; a single Tesla Model 3 battery holds 60–80 kWh. The megawatt-hour (MWh) is used for industrial billing and grid-scale generation.
Calorie (cal) and kilocalorie (kcal) — heat and food
The thermochemical calorie (cal) is fixed at exactly 4.184 J — it was originally the energy needed to raise 1 g of water by 1 °C. The kilocalorie (kcal = 1,000 cal = 4,184 J) is the "Calorie" with a capital C on nutrition labels: a daily intake of 2,000 "calories" really means 2,000 kcal = 8.368 MJ. Confusingly, the lower-case "calorie" on U.S. food packaging actually refers to the kilocalorie. The thermochemical, IT (4.1868 J), and 15 °C calorie definitions differ slightly; this page uses the thermochemical convention, which is the modern default.
British thermal unit (BTU) and therm — North American HVAC
1 BTU = 1,055.05585262 J — the energy to raise 1 lb of water by 1 °F. Air conditioners and heaters in the U.S. are sized in BTU/hour: a window AC is 5,000–24,000 BTU/h, a residential furnace 60,000–120,000 BTU/h. The therm (1 therm = 100,000 BTU = 105.506 MJ) is how natural gas is billed in the United States — a typical home uses 50–100 therms per winter month.
Electronvolt (eV) — atomic and particle physics
1 eV = 1.602176634 × 10⁻¹⁹ J — the energy gained by an electron crossing a 1-volt potential difference. Atomic transitions emit photons of a few eV (visible light is ~1.5–3 eV); chemical bonds break at a few eV; X-rays carry keV; the LHC accelerates protons to 6.5 TeV (= 10¹² eV). The eV is purely a convenience: it keeps subatomic energies as small whole numbers instead of unwieldy joule values like 10⁻¹⁹.
Foot-pound force (ft·lbf) and erg — niche but persistent
1 ft·lbf = 1.3558179483314004 J — the work done lifting a 1 lb weight 1 ft. Still appears in U.S. mechanical engineering and firearm muzzle-energy specifications (a 9 mm round delivers about 460 ft·lbf). The erg (1 erg = 10⁻⁷ J) belongs to the older CGS system and survives in astrophysics, plasma physics, and some thermodynamics texts where the cm-g-s base is convenient.
Real-world energy values across the units
- Food and human metabolism: An apple ≈ 95 kcal = 397 kJ. A slice of pizza ≈ 285 kcal = 1,193 kJ. Daily intake 2,000 kcal = 8.368 MJ = 2.32 kWh. Resting metabolism burns ~80 W ≈ 6.9 MJ/day.
- Household electricity: LED bulb running 1 hour = 0.01 kWh. Microwave 5 minutes = 0.083 kWh. Tumble dryer one cycle = 3 kWh. Average U.S. home = 870 kWh/month. Average UK home = 2,700 kWh/year.
- Heating and cooling: Boiling 1 L of water from room temperature = 335 kJ = 0.093 kWh. Heating an average UK home for one winter ≈ 12,000 kWh. A 12,000 BTU/h portable AC running 8 hours ≈ 28 MJ of heat removed.
- Transport fuel: 1 L of gasoline = 32 MJ = 8.9 kWh. 1 gallon (US) = 121 MJ = 33.7 kWh. A typical car battery (12 V, 60 Ah) holds 0.72 kWh. A Tesla Model 3 long-range battery holds 75 kWh.
- Renewables and grid: 1 m² of rooftop solar in mid-latitudes generates ~150 kWh/year. A 2 MW wind turbine produces ~6,000 MWh/year. The U.S. grid delivered roughly 4 trillion kWh in 2024.
- Atomic and subatomic: Visible-light photon: 2–3 eV. C–H chemical bond: 4.3 eV. X-ray photon: 10 keV. Uranium-235 fission: 200 MeV per atom. LHC proton beam: 6.5 TeV per particle.
- Astrophysics: Solar luminosity = 3.83 × 10²⁶ W (output per second 3.83 × 10³³ erg/s). Type-Ia supernova releases ~10⁵¹ erg = 10⁴⁴ J. The Sun radiates roughly 10²⁶ J/s — energy-density units are inevitable at this scale.
How much is 1 unit of each in joules?
| Unit | Value in joules (J) |
|---|---|
| 1 J (Joule) | 1 Pa |
| 1 kJ (Kilojoule) | 1000 Pa |
| 1 MJ (Megajoule) | 1000000 Pa |
| 1 Wh (Watt-hour) | 3600 Pa |
| 1 kWh (Kilowatt-hour) | 3600000 Pa |
| 1 MWh (Megawatt-hour) | 3600000000 Pa |
| 1 cal (Calorie (thermochemical)) | 4.184 Pa |
| 1 kcal (Kilocalorie (food calorie)) | 4184 Pa |
| 1 BTU (British thermal unit (IT)) | 1055.05585262 Pa |
| 1 therm (Therm (US)) | 105505585.262 Pa |
| 1 eV (Electronvolt) | 1.602176634e-19 Pa |
| 1 ft·lbf (Foot-pound force) | 1.3558179483314004 Pa |
| 1 erg (Erg) | 0.0000001 Pa |
Frequently asked questions about energy units
Why does my food label say "kcal" but the package also says "calories"?
Because the food industry uses "Calorie" (capital C, sometimes uncapitalised on US packs) as a synonym for kilocalorie. 1 dietary Calorie = 1 kcal = 1,000 small calories = 4,184 joules. So a "500-calorie meal" actually contains 500,000 small calories = 500 kcal = 2,092 kJ. The EU and UK now require the kJ value alongside kcal on labels to remove the ambiguity. When in doubt, food contexts always mean kcal.
How much energy is in 1 kilowatt-hour?
1 kWh = 3,600,000 J = 3.6 MJ = 3,412 BTU = 860 kcal. Practically: 1 kWh runs a 1,000-watt appliance for one hour, or a 100-watt bulb for ten hours. It costs roughly $0.15 in the U.S., £0.27 in the UK, or €0.30 in much of the EU as of 2025. Your electricity meter ticks in kWh; multiply by your tariff to estimate any appliance's running cost.
What does BTU mean for an air conditioner?
An AC unit's BTU rating is its cooling capacity per hour — heat removed from the room each hour, in BTU. Rule of thumb: 20 BTU/h per square foot of floor area, so a 300 sq ft bedroom needs 6,000 BTU/h. "BTU" alone in air-conditioner marketing always implies BTU/h. To convert to wattage: divide BTU/h by 3.412 to get watts of cooling power. A 12,000 BTU/h unit is roughly 3.5 kW of cooling.
Are calories and kilocalories interchangeable?
No — they differ by exactly 1,000×. 1 kcal = 1,000 cal = 4,184 J. Only food labels conflate them by writing "calorie" when they mean kilocalorie. Scientific and engineering contexts always use the lower-case "cal" for the small (gram-) calorie and "kcal" for the large (kilogram-) calorie. The thermochemical, IT, and 15 °C calorie definitions also differ slightly from each other (4.184, 4.1868, 4.1855 J) — modern usage settles on 4.184 J unless the standard says otherwise.
Why does 1 BTU equal 1,055 joules and not a round number?
Because the BTU was defined empirically — "the heat needed to raise 1 lb of water by 1 °F" — and that physical quantity is what it is. Several slightly different BTU definitions exist (IT, thermochemical, mean, 60 °F, 39 °F), each fixing the value at a different spot to four or five decimals. The IT BTU (1,055.05585262 J) is the modern engineering default and the value used here.
What's the difference between watt and watt-hour?
Watt is power (rate of energy use); watt-hour is energy (an amount). A 100-W bulb consumes energy at a rate of 100 J/s. Run it for one hour and you've used 100 W × 1 h = 100 Wh = 0.1 kWh. Your electricity bill charges for kWh — the integral of power over time. Battery capacities are also in Wh or mAh (× voltage = Wh).
How much energy does the human body need per day?
About 2,000–2,500 kcal for a moderately active adult — the figure plastered on every food label. That is 8.4–10.5 MJ, or 2.3–2.9 kWh, or 100–120 W of average power output (24 hours). Athletes in heavy training double or triple that. Resting metabolism alone burns ~70–80 W just keeping cells, organs, and brain running.
What is an electronvolt and why is it useful?
An electronvolt (eV) is the energy gained by an electron accelerated through a 1-volt potential difference: 1.602176634 × 10⁻¹⁹ J. It is wildly inconvenient to write atomic-scale energies in joules, so physicists shifted to eV. Visible-light photons carry 1.5–3 eV; X-rays, 100 eV–100 keV; gamma rays, MeV; the LHC, TeV. The factor 1 eV / J is fixed exactly because the elementary charge is now defined exactly (since 2019).
How is heating fuel measured — therms, BTU, or kWh?
It depends on country. U.S. natural-gas bills use therms (1 therm = 100,000 BTU = 105.506 MJ = 29.3 kWh). UK gas bills moved from therms to kWh in 1995 — the meter still measures cubic feet but the bill multiplies by the gas calorific value. EU markets use either kWh or m³ (with calorific value listed). One "therm" of gas heats roughly 95 L of water from 20 °C to boiling.
Can I link to a specific energy conversion?
Yes. The URL updates as you change units and values. Example: ?from=kcal&to=kJ&x=2000 shares a typical daily-intake conversion. Copy the address bar after any conversion to share or bookmark.
References
Dedicated converter pages
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