Energy is the capacity to do work, measured in joules or kilowatt-hours. Power is the rate of energy transfer, measured in watts (joules per second). Think of it like water: energy is the total amount of water in a tank, while power is how fast water flows out of the tank.

Food 'calories' are actually kilocalories (kcal). One food calorie equals 1,000 scientific calories or about 4,184 joules. This is why food labels sometimes show both 'Cal' (kilocalories) and 'cal' (calories), though 'calories' on food packaging always means kilocalories.

Kilowatt-hours (kWh) measure energy consumption over time. A 1,000-watt appliance running for 1 hour uses 1 kWh. Examples: a typical home uses 20-30 kWh/day, a LED bulb (10W) uses 0.01 kWh per hour, while an electric oven (3000W) uses 3 kWh per hour.

BTUs (British Thermal Units) measure thermal energy, commonly used for heating, cooling, and fuel energy content. Air conditioners, furnaces, and water heaters are often rated in BTUs. One BTU is the energy needed to heat one pound of water by one degree Fahrenheit.

An electron volt (eV) is extremely tiny: 1.602176634 × 10⁻¹⁹ joules. It's used in atomic and particle physics because it represents the energy scale of atomic processes. X-rays have keV (1,000 eV) energy, while chemical bonds involve several eV. Nuclear processes use MeV (1,000,000 eV) and GeV (1,000,000,000 eV).

Different fuels have different energy densities. Gasoline contains about 33.7 kWh per gallon, natural gas about 29.3 kWh per 100 cubic feet (CCF), and heating oil about 40 kWh per gallon. This helps compare fuel costs and efficiency.

When you burn 100 food calories through exercise, you're expending 100 kcal or about 418,400 joules of energy. This is roughly equivalent to the energy in 1.2 watt-hours, or enough to power a LED light bulb for about 5 hours.

Energy conversion efficiency varies widely: LED bulbs are ~90% efficient (10% becomes heat), gasoline cars ~25%, electric cars ~90%, solar panels ~20%, and hydroelectric plants ~90%. Understanding efficiency helps evaluate true energy costs.

Use Megajoules (MJ) and Gigajoules (GJ) for large-scale energy measurements like industrial processes, building energy consumption, or fuel energy content. Megawatt-hours (MWh) and Gigawatt-hours (GWh) are used for electrical energy production and consumption - power plants, grid capacity, and large-scale renewable energy projects. 1 MWh = 3.6 GJ, and 1 GWh = 3,600 GJ.

Therms (thm) measure natural gas energy - 1 therm = 100,000 BTU, commonly used for gas bills and heating costs. Mechanical energy units include foot-pounds (ft⋅lb) for torque and work in imperial systems, inch-pounds (in⋅lb) for smaller mechanical work, and kilogram-force meters (kgf⋅m) for metric mechanical energy. These are used in engineering, automotive, and mechanical applications.