Energy Unit Converter

The small calorie (cal) is defined as the energy needed to raise the temperature of 1 gram of water by 1°C. The food calorie, properly called kilocalorie (kcal), is 1,000 times larger and raises 1 kilogram of water by 1°C. When you see “Calories” on food labels, these are actually kilocalories. This distinction is crucial in nutrition and scientific contexts. For example, a candy bar labeled as “250 Calories” contains 250 kcal or 250,000 cal. There are also two slightly different definitions of calories: thermochemical calories and International Table calories, which differ by about 0.06%.

Electronvolts (eV) are primarily used in atomic and nuclear physics where energies involved are extremely small by everyday standards. One eV is the energy gained by an electron moving across an electric potential difference of one volt. While a joule is suitable for macroscopic energies (like lifting an apple), it would be unwieldy for atomic processes—a single joule equals approximately 6.242×10¹⁸ eV. Electronvolts are commonly used when discussing: binding energies of atoms and molecules, energy levels in atoms, particle physics reactions, semiconductor band gaps, and X-ray or gamma ray energies. For example, visible light photons have energies of about 1.5-3.0 eV, while a typical chemical bond might have an energy of a few eV.

The joule (J) is the basic SI unit of energy, while the kilowatt-hour (kWh) is commonly used for electrical energy, especially in utility billing. One kilowatt-hour equals 3,600,000 joules or 3.6 megajoules. The relationship comes from the definition: a watt is one joule per second, so a kilowatt is 1,000 joules per second. When you use one kilowatt of power for one hour (3,600 seconds), you’ve used 1,000 × 3,600 = 3,600,000 joules of energy. Practically speaking, 1 kWh is roughly the energy needed to run a 1,000-watt appliance (like a small space heater) for one hour, while 1 joule is approximately the energy needed to lift a small apple one meter against Earth’s gravity.

British Thermal Units (BTU) remain in common use despite metrication primarily in heating, cooling, and air conditioning (HVAC) industries, particularly in the United States. A BTU is traditionally defined as the energy needed to raise one pound of water by one degree Fahrenheit. BTUs are convenient for several reasons: (1) Historical precedence and established industry standards in HVAC systems, with vast amounts of literature and reference material using BTU; (2) Practical sizing in building applications—a typical home heating system might be rated at 60,000-100,000 BTU/hr, which is more intuitive than megajoules; (3) Regulatory standards in the US still commonly use BTU for energy efficiency ratings. While scientific work typically uses joules, the expense and difficulty of changing established practices keeps BTU relevant in certain industries.

Energy conversion units directly relate to the law of conservation of energy, one of physics’ fundamental principles stating that energy can neither be created nor destroyed, only converted between forms. When we convert between units like joules, calories, and BTUs, we’re simply using different measurement scales for the same fundamental quantity—energy. This principle explains why precise conversion factors exist between all energy units. For example, when 4.184 joules of electrical energy becomes heat energy, it equals exactly 1 calorie of thermal energy—no energy is lost, just transformed. While our converter focuses on mathematical unit conversions, practical energy conversions (like electricity to heat in a stove) always follow this conservation law, though real-world inefficiencies mean some energy converts to unintended forms (like sound or waste heat), even though the total energy remains constant.