The Language of Energy and Power
Understanding and converting between different units is essential in energy calculations.
Units of Energy
Energy is the capacity to do work.
Joule (J): The SI unit of energy. It is a small unit (1 J is the energy to lift a small apple 1 meter).
Calorie (cal): Originally the energy needed to raise 1 gram of water by 1°C. 1 cal ≈ 4.184 J.
Kilocalorie (kcal or Calorie): The 'food calorie' is 1000 calories. 1 Cal = 1 kcal = 4184 J.
British Thermal Unit (BTU): The energy needed to raise 1 pound of water by 1°F. 1 BTU ≈ 1055 J.
Kilowatt-hour (kWh): A common unit for billing electricity. It is the energy consumed by a 1 kilowatt device running for 1 hour. 1 kWh = 3.6 million J.
Units of Power
Power is the rate at which energy is used or transferred.
Watt (W): The SI unit of power. 1 Watt = 1 Joule per second (1 J/s).
Kilowatt (kW): 1000 Watts.
Horsepower (hp): A non-SI unit still common for engines. 1 hp ≈ 746 W.
The Core Relationship: Power, Energy, and Time
The fundamental relationship connecting these quantities is:
Energy = Power × Time
This allows for conversion and calculation.
If you know the power of an appliance in Watts and the time it runs in seconds, you can calculate the total energy used in Joules.
If your electric bill shows you used 300 kWh of energy in a month, you can calculate the average power you consumed.
Example Calculation: A 100 W light bulb is left on for 10 hours. How much energy did it consume in kilowatt-hours?
1.Convert Power to kW: 100 W = 0.1 kW.
2.Use the formula: Energy = Power × Time
Energy = 0.1 kW × 10 hours = 1.0 kWh.