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Understanding Energy Units
What is Energy?
Energy is the capacity to do work. It can exist in many forms: kinetic (motion), potential (stored), thermal (heat), electrical, chemical, and nuclear. Energy is conserved - it can be converted from one form to another but never created or destroyed.
Common Energy Units
Joule (J)
The SI unit of energy. One joule is the work done when a force of one newton moves an object one meter. Named after James Prescott Joule.
Kilowatt-hour (kWh)
Common unit for electrical energy. Your electricity bill is measured in kWh. 1 kWh = 3,600,000 joules.
Calorie (cal)
Energy needed to raise 1 gram of water by 1°C. The "Calorie" on food labels is actually a kilocalorie (kcal).
BTU
British Thermal Unit. Common in HVAC systems. Energy to raise 1 pound of water by 1°F.
Calories vs Calories
Small vs Large Calorie
There are two types of calories:
Small calorie (cal): Energy to heat 1g of water by 1°C
Large Calorie (Cal or kcal): Energy to heat 1kg of water by 1°C
Food labels use the large Calorie (kcal), even though they write "Calories." A 100-Calorie snack contains 100,000 small calories.
Energy Comparisons
| Energy Source/Activity | Energy |
|---|---|
| AA battery | ~10 kJ |
| Food Calorie | 4.184 kJ |
| Running for 1 hour | ~2,500 kJ |
| Daily food intake (2000 Cal) | ~8,400 kJ |
| 1 gallon of gasoline | ~120 MJ |
| Average household/day (US) | ~100 MJ |
| Ton of TNT | ~4.2 GJ |
Energy vs Power
Understanding the Difference
Energy is the total amount of work that can be done (measured in joules, kWh).Power is the rate at which energy is used (measured in watts, where 1 W = 1 J/s).
Example: A 100W light bulb uses 100 joules per second. Running it for 10 hours uses 1 kWh of energy (100W × 10h = 1000Wh = 1 kWh).
Conversion Formulas
kWh = joules ÷ 3,600,000
kcal = joules ÷ 4,184
BTU = joules ÷ 1,055
eV = joules ÷ 1.602 × 10⁻¹⁹
The Electronvolt
The electronvolt (eV) is used in atomic and particle physics. It's the energy gained by an electron accelerating through 1 volt potential difference. While tiny (1.6 × 10⁻¹⁹ J), it's convenient for atomic-scale measurements. Particle energies are often in MeV, GeV, or TeV.
Frequently Asked Questions
What is the difference between a calorie and a Calorie?
A lowercase 'calorie' (cal) is the energy needed to raise 1 gram of water by 1°C. An uppercase 'Calorie' (Cal), also called a kilocalorie (kcal), equals 1,000 small calories. Food labels use Calories (kcal), so a 100-Calorie snack actually contains 100,000 small calories or about 418,400 joules.
How do I convert kilowatt-hours to joules?
Multiply kilowatt-hours by 3,600,000 to get joules. One kWh equals 3.6 megajoules (MJ). For example, if your appliance uses 2 kWh, that's 7,200,000 joules or 7.2 MJ of energy.
What is a BTU and when is it used?
BTU (British Thermal Unit) is the energy needed to raise 1 pound of water by 1°F. It equals about 1,055 joules. BTUs are commonly used in the US for rating heating and cooling systems (HVAC), water heaters, and measuring natural gas consumption.
What's the difference between energy and power?
Energy is the total capacity to do work (measured in joules, kWh, calories), while power is the rate of energy transfer (measured in watts). A 100-watt light bulb uses 100 joules per second. Running it for 10 hours uses 1 kWh of energy (100W × 10h = 1000Wh).