Energy Conversion Guide

Energy is the capacity to do work. The SI unit is the joule (J), defined as the work done by a force of one newton acting over one metre. Energy comes in many forms — kinetic, potential, thermal, chemical, electrical — but they share a unit because all forms convert into work to within the laws of thermodynamics. A litre of petrol stores about 33 megajoules of chemical energy; a person burns about 8–10 megajoules per day in basal metabolism; a household microwave at 1,000 W running for 5 minutes consumes 300 kilojoules.

Energy is not power. Energy is what you use; power is the rate at which you use it. A 100-watt bulb burning for one hour consumes 100 watt-hours, which is 360,000 joules. The bulb's power is 100 W; the energy delivered is 360 kJ. The kilowatt-hour on your electricity bill is energy, not power, even though the word 'kilowatt' is right there in the name.

The joule was named for James Prescott Joule, the 19th-century English physicist who pinned down the mechanical equivalent of heat through stirring-paddle experiments. Joule's value (~4.155 J/cal) was within 1% of the modern 4.184 J/cal. The unit joined the SI catalogue in 1960.

The calorie predates the joule. It was defined as the heat required to raise one gram of water by one degree Celsius (more precisely, at 14.5 to 15.5 °C). Three calorie variants appeared in the 20th century: the thermochemical calorie (4.184 J exactly), the IT calorie (4.1868 J), and the mean calorie (4.190 J). All three are still in scientific literature. Our converter uses the thermochemical calorie, the most common modern value, and the corresponding kilocalorie at 4,184 J exactly.

The British Thermal Unit (BTU) is the imperial analogue of the calorie: the energy to raise one pound of water by one degree Fahrenheit. It also has multiple definitions — the IT BTU at 1,055.05585 J (the value we use), the thermochemical BTU at 1,054.350 J, the 39 °F BTU, the 60 °F BTU. The 0.07% spread between IT and thermochemical BTU is rarely noticed in HVAC catalogues, where the published rating is treated as a round number.

The kilowatt-hour entered everyday use through electricity billing in the late 19th century. It is a derived unit: 1 W × 1 h = 1 W × 3,600 s = 3,600 J. The kilowatt-hour is therefore 3,600,000 J = 3.6 megajoules exactly. The unit survived into the 21st century because it gives a comfortable three- or four-digit number on a monthly residential bill.

Our energy category supports nine units. SI: joule, kilojoule, megajoule. Heat: calorie (1 cal = 4.184 J, the small or thermochemical calorie), kilocalorie (1 kcal = 4,184 J — the unit on food labels, usually shortened to 'calorie' or 'Cal'). Electrical: watt-hour (1 Wh = 3,600 J), kilowatt-hour (1 kWh = 3,600,000 J). Imperial heat: British Thermal Unit (1 BTU = 1,055.05585 J, the IT BTU). Particle physics: electron volt (1 eV = 1.602176634 × 10⁻¹⁹ J, exact by the 2019 SI redefinition).

We use the IT BTU (1,055.05585 J), which is the value adopted by ISO 31-4 and the most common in HVAC literature. Older US thermochemical BTU values (1,054.350 J) are about 0.07% smaller; the difference is generally below the precision of furnace ratings.

Every energy conversion routes through the joule, the SI base. Each unit declares how many joules it represents: 1 cal = 4.184 J, 1 kcal = 4,184 J, 1 Wh = 3,600 J, 1 kWh = 3,600,000 J, 1 BTU = 1,055.05585 J, 1 eV = 1.602176634 × 10⁻¹⁹ J. The methodology page lists every factor.

Useful round-number conversions: 1 kWh = 3.6 MJ exactly. 1 kWh ≈ 860 kcal. 1 kWh ≈ 3,412 BTU. 1 kcal ≈ 3.97 BTU. A daily food intake of 2,000 kcal is 8.4 MJ — the basis for European nutrition labels, which list kJ alongside kcal. A typical US home uses 30 kWh per day, which is roughly 108 MJ or 102,000 BTU.

Cooking, fitness, and utility bills drive most energy searches. Kilowatt-hours to British Thermal Units covers heating-cost comparisons between electric and gas — natural-gas utilities sell therms (100,000 BTU each), electric utilities sell kWh, and the relationship is 1 kWh = 3,412 BTU. Joules to calories and calories to joules handle physics-class conversions. Kilocalories to joules is the food-label conversion most commonly searched in Europe, where labels show both kJ and kcal. Joules to kilowatt-hours handles battery-capacity figures: a 100 Wh laptop battery stores 360,000 J or 0.1 kWh.

The full energy comparison table lists every supported unit with its joule factor and a worked sample. Use it when you need to compare a heating bill in therms, a battery in Wh, and a recipe's calorie count at the same time.

Food 'calorie' means kilocalorie: a 250-calorie cookie has 250 kcal = 1,046 kJ of energy, not 250 cal. The convention dates from the 19th century and is now baked into food labels worldwide. If you see 'calorie' on a nutrition label, multiply by 1,000 to get joules. Some scientific literature uses 'Cal' (capital C) for kilocalorie and 'cal' (lowercase) for the small calorie, but this distinction is not enforced and many writers conflate them.

Kilowatt-hour is energy, not power: a 5 kWh battery stores 5 kilowatt-hours of energy; it can deliver 5 kW for one hour, or 1 kW for five hours, or 100 W for 50 hours. The 'kilowatt' in kilowatt-hour names the power, not the energy. Mixing the two leads to capacity errors that compound across solar-and-storage spec sheets.

BTU has multiple definitions: the IT BTU we use is 1,055.05585 J; the thermochemical BTU is 1,054.350 J; the 39 °F BTU is 1,059.67 J. The IT BTU is the standard in modern HVAC and most international literature, but US-only ratings sometimes use a different one. The 0.5% spread rarely matters for furnace sizing, but it can show up in research papers and standards documents.

Watt-hour vs joule: 1 Wh = 3,600 J. People who think in watts and seconds get this right; people who think in 'kilo' SI prefixes occasionally drop a factor of 3.6 because the metric ladder doesn't apply between joules and watt-hours.

Electron volts in chemistry vs physics: 1 eV per molecule equals 96.485 kJ/mol — the Faraday constant, useful for bond energies. The eV is everywhere in semiconductor physics (silicon bandgap ≈ 1.12 eV) and rare in chemistry (where kJ/mol is standard). The two communities sometimes report the same quantity in different units, separated by the Faraday-constant conversion.

Side-by-side: the full energy comparison table lists every supported unit with its factor and a worked sample. Method and sources: the methodology page cites NIST SP 811. The IT calorie (4.1868 J) and IT BTU (1,055.05585 J) values used in our engine match the ISO 31-4 conventions widely adopted in HVAC and thermodynamics literature. Context: our piece on energy units decoded covers calories, kilowatt-hours, BTU, and the food-label convention with worked examples.