Energy Converter

Energy Converter — Convert Joules, kWh, Calories, BTU & More

The Energy Converter by Toolsiro converts between 13 energy units instantly — Joule, Kilojoule, Megajoule, Calorie, Kilocalorie, Watt-hour, Kilowatt-hour, Megawatt-hour, Electronvolt, BTU, Erg, Therm, and Foot-pound. Every conversion updates live as you type, and clicking any result card sets it as your new input for chained conversions.

Energy is one of the most fundamental quantities in physics and one of the most practically important in everyday life — from the calories in food to the kilowatt-hours on your electricity bill to the BTUs in your air conditioner. The diversity of energy units reflects the historical development of different fields, each adopting the most convenient unit for its specific range of applications.

The Joule — The Scientific Standard

The Joule (J) is the SI unit of energy, defined as the work done when a force of one Newton acts through a distance of one meter. Named after James Prescott Joule, the English physicist who established the relationship between heat and mechanical work in the 1840s, it is the foundation unit from which all other energy conversions derive.

One Joule is a surprisingly small amount of energy in everyday terms. Lifting a 100-gram object one meter requires about 1 Joule. The kinetic energy of a tennis ball moving at 25 m/s is about 3.5 Joules. A food calorie (kilocalorie) is 4,184 Joules. For large-scale energy — electricity, heating, industrial processes — kilojoules (kJ) and megajoules (MJ) are more practical. One kilowatt-hour equals 3.6 megajoules.

Kilowatt-Hour — Your Electricity Bill's Unit

The kilowatt-hour (kWh) is the practical unit of electrical energy for consumers and the standard unit on electricity bills worldwide. One kWh is the energy consumed by a 1,000-watt (1 kW) device running for exactly one hour. It equals 3,600,000 Joules (3.6 MJ).

To put kWh in perspective: a standard LED light bulb (10 watts) running for 100 hours uses 1 kWh. A typical washing machine cycle uses about 0.5–2 kWh. An electric vehicle might use 15–25 kWh per 100 kilometers. The average American household consumes about 900 kWh per month. An average European household uses about 300–500 kWh per month.

The megawatt-hour (MWh) is used for industrial energy consumption and power plant output. A medium-sized power plant might generate hundreds of MWh per hour. Large data centers consume tens of thousands of MWh per year.

Calories and Kilocalories — The Food Energy Units

The calorie (cal) is defined as the amount of energy needed to raise the temperature of 1 gram of water by 1°C. This is a small unit — one food calorie is actually a kilocalorie (kcal), equal to 4,184 Joules. The confusion between the small calorie and food calorie is longstanding and deliberate: nutritional labels use "Calorie" (capital C) to mean kilocalorie, while "calorie" (lowercase) means the small unit.

A typical adult requires 1,600 to 2,500 kcal (food Calories) per day depending on size, age, and activity level. One gram of carbohydrate provides 4 kcal, one gram of protein provides 4 kcal, and one gram of fat provides 9 kcal — which is why fats are such a dense energy source. A Big Mac contains about 550 kcal (2,300 kJ). Running a kilometer burns approximately 60–80 kcal depending on body weight and pace.

BTU — The Heating and Cooling Standard

The British Thermal Unit (BTU) is the energy unit of the US heating and cooling industry. One BTU is defined as the energy needed to raise the temperature of one pound of water by 1°F — approximately 1,055 Joules. Despite the "British" name, BTU is used almost exclusively in the United States today, while the UK has largely switched to SI units.

Air conditioner and heater capacities are rated in BTU per hour (BTU/h). A small window air conditioner might be rated at 5,000 BTU/h, while a central HVAC system for a large home might be 60,000 BTU/h or more. Natural gas is sold by the therm in the US — one therm equals 100,000 BTU or approximately 29.3 kWh. Home heating oil is measured in BTU content per gallon.

Electronvolt — The Atomic Scale Unit

The electronvolt (eV) is an extremely small unit used in atomic physics, nuclear physics, particle physics, and chemistry to express the energies of subatomic particles and photons. One electronvolt is the kinetic energy gained by an electron when accelerated through a potential difference of one volt — approximately 1.602 × 10⁻¹⁹ Joules.

Visible light photons have energies of 1.8 to 3.1 eV. Chemical bond energies are typically 1 to 10 eV. X-ray photons carry energies of thousands to hundreds of thousands of eV (keV). The Large Hadron Collider accelerates protons to energies of 6.8 × 10¹² eV (6.8 TeV). Despite being tiny, the electronvolt makes atomic-scale energy calculations manageable by avoiding the scientific notation required when working in Joules.

Erg — The CGS Unit

The erg is the energy unit of the centimeter-gram-second (CGS) system, an older measurement system that preceded SI. One erg equals 10⁻⁷ Joules — an incredibly small unit even by scientific standards. The erg was once common in physics and astronomy but has largely been replaced by the Joule in modern scientific literature. It is still occasionally encountered in older texts and in some astrophysics contexts where expressing energies in ergs per second (erg/s) for luminosity remains traditional.

Foot-Pound and Therm

The foot-pound (ft·lb) is the energy unit of the US customary and imperial systems, equal to the work done by a force of one pound-force acting through a distance of one foot — approximately 1.356 Joules. It appears in automotive specifications (engine torque in lb·ft is technically a torque unit, not energy, but the same conversion applies) and mechanical engineering contexts in the United States.

The therm is a larger unit used in the natural gas industry in the United States. One therm equals exactly 100,000 BTU, or approximately 105.5 MJ or 29.3 kWh. Natural gas utility bills in the US commonly report consumption in therms, making it important for energy cost calculations.

Energy Efficiency and Conversion Losses

Energy conversion between different forms is never perfectly efficient. A coal power plant converts about 33–40% of the chemical energy in coal into electrical energy. A natural gas combined cycle plant achieves 50–60% efficiency. An electric motor converts 85–95% of electrical energy into mechanical work. A gasoline engine converts only 20–40% of fuel energy into motion, with the rest lost as heat. LED bulbs convert about 90% of electrical energy into light, while incandescent bulbs convert only 5–10%.

These efficiency factors are crucial when comparing energy costs across different technologies. The 3.6 MJ per kWh conversion factor becomes important when comparing the energy content of fuels to electricity prices, or when calculating the environmental impact of different energy sources.

Frequently Asked Questions

How many joules in a kilowatt-hour? Exactly 3,600,000 Joules (3.6 MJ). This is because 1 kW = 1,000 W = 1,000 J/s, and 1 hour = 3,600 seconds, so 1 kWh = 1,000 × 3,600 = 3,600,000 J.

What's the difference between a calorie and a Calorie? A calorie (lowercase, cal) is 4.184 Joules — the energy to heat 1g of water by 1°C. A Calorie (uppercase, kcal on nutrition labels) is 1,000 calories = 4,184 Joules. Food labels always use kilocalories.

How many BTU in a kWh? Approximately 3,412 BTU per kWh. This conversion is important for comparing heating costs between natural gas (sold in therms or BTU) and electricity (sold in kWh).

What unit should I use for my electricity bill calculations? Kilowatt-hours (kWh) — that's what utility companies charge for and the unit on your electricity meter. For large industrial calculations, megawatt-hours (MWh) are more convenient.