Energy Converter

Energy Converter

Convert between all energy units: joules (SI standard), calories and food Calories, kilowatt-hours (electrical billing), BTU (HVAC), and electron volts (particle physics). Energy is the same physical quantity whether it comes from food, fuel, electricity, or nuclear reactions — only the units differ.¹

Energy Unit Conversions

How Energy Units Relate to Each Other

Energy is a single physical quantity — the capacity to do work — but different fields measure it in wildly different units because of historical convention and practical scale. A joule is the SI base unit (one newton applied over one meter), but physicists working at atomic scales prefer electron volts (eV), nutritionists use food Calories (kilocalories), engineers measure electricity in kilowatt-hours (kWh), and HVAC professionals rely on British Thermal Units (BTU). Converting between them is straightforward because each unit represents a fixed number of joules, but the conversion factors span many orders of magnitude — a single food Calorie equals 4,184 joules, while one kWh equals 3.6 million joules.

Complete Energy Conversion Table

Energy in Everyday Life

Putting energy units into tangible perspective helps make these conversions meaningful. A slice of bread contains roughly 70 food Calories — equivalent to 293,000 joules or 0.081 kWh. That means the energy in one slice of bread could power a 10-watt LED bulb for over 8 hours. A gallon of gasoline contains approximately 120,000 BTU (about 34 kWh), enough to drive a typical car 25–30 miles but enough to power the average American home for more than a day. A smartphone battery holds about 10–15 Wh (36,000–54,000 joules), roughly the energy content of a single almond. These comparisons illustrate why fossil fuels remain so dominant — their energy density per unit weight and volume far exceeds what batteries currently achieve.

Energy Units in Science and Engineering

In thermodynamics and chemistry, the calorie (lowercase) measures energy at the molecular level — it is the energy needed to raise one gram of water by one degree Celsius. The food Calorie (uppercase, or kilocalorie) is 1,000 times larger and appears on nutrition labels. This naming confusion has caused countless errors — always check whether a source means calories or kilocalories. In electrical engineering, the kilowatt-hour dominates because it directly relates power (kilowatts) to time (hours), making it intuitive for billing purposes. Your electric meter literally counts kWh consumed. In particle physics, the electron volt is practical because individual particles carry energy on the order of eV, keV, or MeV — expressing these in joules would require unwieldy scientific notation for every calculation.

BTU and HVAC Applications

The British Thermal Unit remains the standard in heating, ventilation, and air conditioning because the entire industry infrastructure — equipment ratings, ductwork sizing, and building codes — was built around it. An air conditioner rated at 12,000 BTU/hr (one "ton" of cooling) removes 12,000 BTU of heat per hour. Natural gas is billed in therms (100,000 BTU) or MCF (roughly 1,030,000 BTU per thousand cubic feet). When comparing heating fuels, BTU provides a common denominator: natural gas delivers about 100,000 BTU per therm, propane about 91,500 BTU per gallon, heating oil about 138,500 BTU per gallon, and a kWh of electricity delivers 3,412 BTU. Use our AC BTU Calculator to size cooling systems for your space.

Food Energy and Metabolism

The human body converts food energy with surprising efficiency — roughly 25% of metabolic energy becomes mechanical work (muscle movement), while the remaining 75% generates heat that maintains body temperature. A moderately active adult burns approximately 2,000–2,500 food Calories per day, equivalent to about 2.3–2.9 kWh — roughly the energy consumed by a single 100-watt incandescent light bulb running for a day. Macronutrients differ in energy density: fat provides 9 Calories per gram (37.7 kJ/g), protein and carbohydrates each provide 4 Calories per gram (16.7 kJ/g), and alcohol provides 7 Calories per gram (29.3 kJ/g). These conversion factors are the foundation of all dietary calorie counting. For nutrition tracking, see our Calorie Calculator and BMR Calculator.

Electricity Costs and Energy Conversion

Understanding kWh is essential for managing electricity costs. The average U.S. residential electricity rate is approximately $0.16 per kWh, though rates vary from $0.10 in states with cheap hydroelectric or natural gas to over $0.30 in Hawaii and parts of the Northeast. To estimate any appliance's annual electricity cost, multiply its wattage by hours of daily use, divide by 1,000 to get kWh, then multiply by your rate and by 365 days. A 1,500-watt space heater running 8 hours per day costs approximately $700 per year at $0.16/kWh. Use our Appliance Energy Cost Calculator for precise estimates.

How to Use This Converter

1. Enter a value — Type the energy amount you want to convert in any supported unit.
2. Select your units — Choose the source unit and target unit from the dropdown menus.
3. Read the result — The conversion appears instantly with the full conversion factor displayed for reference.

Tips and Best Practices

See also: AC BTU Calculator · Appliance Energy Cost · Calorie Calculator · Unit Converter

Renewable Energy and Unit Conversions

Solar panels are rated in watts under standard test conditions (STC), and their output is measured in kWh over time. A typical residential solar panel produces 350–400 watts at peak and generates 1.2–1.8 kWh per day depending on location, orientation, and shading. A 6 kW rooftop system in a sunny U.S. state produces approximately 8,000–10,000 kWh per year — roughly equivalent to 27–34 million BTU or the energy content of about 280 gallons of gasoline. Wind turbines are similarly rated in kilowatts or megawatts, with a 2.5 MW commercial turbine generating 6,000–8,000 MWh annually at a typical 30% capacity factor. Understanding these energy unit relationships is essential for comparing the output and economics of different renewable energy technologies.

Battery and Electrical Storage Units

Battery capacity is rated in watt-hours (Wh) or milliamp-hours (mAh), and understanding the conversion is critical for comparing devices. A smartphone battery at 4,000 mAh and 3.7V nominal voltage stores 14.8 Wh (53,280 joules). A Tesla Model 3 battery at 60–82 kWh stores the equivalent of about 2 gallons of gasoline in raw energy terms, though electric motors convert this energy to motion about 3 times more efficiently than internal combustion engines. Home battery systems like the Tesla Powerwall store 13.5 kWh — enough to power critical loads (lights, refrigerator, internet) for 8–12 hours during an outage. For whole-home backup at full electrical load, most households would need 3–4 Powerwalls. See our Electricity Bill Calculator for monthly cost estimates and our Energy Savings Calculator to evaluate efficiency upgrades.

Nuclear and Chemical Energy Scales

At the extreme ends of the energy spectrum, the units become dramatically different. A single uranium-235 fission event releases approximately 200 MeV (million electron volts), or about 3.2 × 10⁻¹¹ joules — minuscule individually, but one kilogram of U-235 contains roughly 2.56 × 10²⁴ atoms, so complete fission yields about 82 terajoules (82 × 10¹² J) or approximately 22,800 MWh — enough to power 2,000 homes for a year. On the chemical energy side, common fuels range widely: hydrogen delivers 142 MJ per kilogram (the highest energy density by weight of any chemical fuel), gasoline provides about 46 MJ/kg, coal delivers roughly 24 MJ/kg, and wood provides approximately 16 MJ/kg. These figures explain why hydrogen is the fuel of choice for space propulsion and why nuclear power produces extraordinary energy from very small quantities of fuel — a single uranium fuel pellet the size of a pencil eraser produces as much energy as one ton of coal, 17,000 cubic feet of natural gas, or 149 gallons of oil.

These energy-scale comparisons are essential for policy discussions around energy transition, grid storage requirements, and decarbonization strategies where different generation sources must be compared on equal footing.

Whether you are comparing electricity rates, sizing a solar installation, evaluating heating fuel options, or calculating dietary energy needs, understanding energy unit relationships is an essential literacy for informed decision-making in a world that runs on energy.

How to Use This Calculator

1. Enter energy value — Amount to convert.
2. Select units — From and to.
3. See result — Instant with conversion factor.

Tips and Best Practices

See also: Electricity Cost · Calorie · Temperature · Pressure