Energy Concepts - Units
Energy Units
Energy is measured in Btu, an abbreviation for British Thermal Unit. The Btu is defined as the heat to raise one pound of water one degree fahrenheit. Because of the definition, the Btu is used mostly as a unit of heat, the thermal form of energy. All forms of energy can use the unit Btu. However, other forms of energy tend to have their own units.
Mechanical energy has the foot-lb (ft-lbf). |
Electrical energy has the watt-hour(w-hr). |
Chemical energy has the calorie. |
The SI (International System of Units) has the joule (j). |
The above are all different units for energy and can be converted from one to the other by using a conversion factor.
Some units are expressions for larger amounts of energy than the Btu. A 1000 Btu can be abbreviated as MBtu. A point of confusion, the "M" is not the metric abbreviation M. A 100,000 Btu can be called a therm. Your natural gas bill may refer to your gas consumption in therms. A 1,000,000 Btu can be abbreviated MMBtu. When referring to the annual energy consumption of the US, the unit quad may be used. This stands for a quadrillion Btu, which is 10 to the 15 power Btu or 1000 trillion or a million billion Btu.
Electrical energy is the SI (International System of Units) joule. However due to the popularity of the unit watt, which is power, the watt-second or watt-hour is more common. A 100 watt light bulb energized for one hour produces 100 watt-hours of energy.
Remember energy is a specific quantity. Your electric bill and you natural gas bill charges you for the energy you use. Your electric bill shows the kilowatt-hours (kwh), you use. A kilowatt-hour is 1000 watt-hours. Your gas bill will be either therms or ccf, which is 100 cubic feet. Your gas company either bills base on actual cubic feet used or the energy of that amount.
Power Units
When talking about energy, you also need to specify a time interval. The average person in the United States used 334 million Btu in 2006. This statement shows the amount of energy used over one year's time. The amount of energy at a given moment is power. Power is the rate of flow of energy.
The unit of power is the Btu/hr, btu per hour. Energy providing or consuming devices are often rated in power units. A furnace may be rated at 80,000 Btu/hr. This means if the furnace runs for an hour it will use 80,000 Btu. If the furnace runs only 5 minutes, then it will use only 6667 Btu. You may see a rating in btuh; this is the same as Btu/hr.
You may have seen air conditioners rated in tons. This is not its weight; it is the power of the unit. A ton of air conditioning is 12,000 Btu/hr. The term "ton" comes from when ice was used for cooling, the ice was purchased by weight.
For electricity, power is usually expressed in watt or kilowatt. A kilowatt is 1000 watts. Electrical appliances and devices are rated in watts. A light bulb may be rated at 60 watts, a toaster may be rated at 1000 watts. To obtain the energy they use, multiply the power (rating) by the time it operates. A caution here is that the rating may be peak power and not always the power while the device operates. Electronic devices like DVD players will only use near its rated power when the disk is spinning at its highest speed. Other devices like ovens turn the element on and off. Bottom line is be careful on how you calculate the energy from rated power.
Mechanical power uses the unit called horsepower. A horsepower (hp) is 746 watts. Due to the higher power of engines, horsepower is in common use. Also large motors are rated in horsepower. BTW, the term horsepower was introduced by James Watt, who is honored by the term watt.
Another important caution, do not convert the rating of an air conditioner into watts in order to determine its electrical power. Go to the air conditioning pages for an explanation of this common mistake.
Try to keep clear the difference between energy and power. A short cut to remember the difference, is energy is a set quantity. Like the amount of energy you are billed by your electric company. A gallon of gasoline has 125,000 Btu. This is energy.
Power is a capacity or capability. A 10 Megawatt power plant has a maximum capacity of 10 million watts. A light bulb is rated at 100 watts. A motor is rated at 10 horsepower. This is power.
Go to the next pages to find more detailed information about heat transfer.
