Because there are many forms of energy, there are many ways to measure it. Unfortunately, this has given rise to different units for energy, and these remain in common usage.

• The standard unit in the metric system is the Joule, which is the amount of energy it takes to raise a 102 gram object 1 meter -- so lifting a 1 liter bottle of soda from the floor to the middle shelf of the refrigerator costs you 10 Joules. We can see that a Joule is not a lot of energy (lifting a 102 kilogram object 1 meter requires 1000 Joules; some people do this just to get out of bed!).
  Using British units, you would lift 1 pound a distance of one foot, and this is called a "foot-pound"; it's a little more than a Joule. You will occasionally hear this unit mentioned, but it is obsolete. On the electrical bill, energy is measured in Kilowatt-hours (KWhr), which is 3,600,000 Joules. A Kilowatt-hour is enough energy to lift you about 3 miles; the power company charges 5 cents for 1 Kilowatt- hour.

  (Here is how it gets its name: people are also interested in the rate of delivery of energy; this concept is called power and is measured in Watts. 1 Watt means delivering 1 Joule every second, and a Kilowatt is 1000 Joules every second. Since there are 3600 seconds in an hour, we receive 3,600,000 Joules in an hour from a 1 Kilowatt power supply).

• On the gas bill energy is measured in therms. A therm is 100,000 British Thermal Units (BTU). This unit is defined in terms of heating something, instead of lifting it; this is how there came to be another unit in the first place. The BTU is 1055 Joules, and so a therm is 105,500,000 Joules. It takes a lot of energy to heat a house, and so it needs a big unit!
• There is also a metric system unit based on heating something, called the calorie. It is worth 4.2 Joules. But the unit that is used to measure energy in food -- the calories that you count -- is actually a kilocalorie = 1000 calories = 4200 Joules. One teaspoonful of sugar is 15 (Kilo)calories = 63,000 J -- enough to lift a rather heavy person out of bed and onto his feet 63 times. (People are not very efficient as machines: it takes a lot more energy input to accomplish something than is accounted for by the mechanical work that is done. Climbing a 20 story building might increase your gravitational energy by 63,000 J, but will require more than 15 Kcal of food. The rest of the energy goes to making you hot and getting your heart pounding.)
• At the gasoline pump, energy is measured in gallons. One gallon of gasoline is worth 135,000,000 J.

Check the box when you are done:   Discussion of room temperature