1. What “units” do we use to measure LENGTH?inches, feet, miles, meters, kilometers... MASS?pounds, ounces, grams, kilograms... TEMPERATURE? degrees F, degrees C … ENERGY?Joules, calories, BTU *, kWh * … *British Thermal Units, kilowatt-hours

2. Which contains more ENERGY, a barrel of oil or a ton of coal? “Units” are necessary to be able to measure and compare things of interest:

3. 4.19 Joules =1 calorie 252 calories = 1 BTU 3,412 BTU =1 kWh 5,800,000 BTU =1 Barrel oil 25,200,000 BTU = 1 T of coal 3,600,000 Joules =1 kWh

4. POWER is often confused with ENERGY POWER = the amount of ENERGY used per unit of time. Joules per second kWh per month BTU per year … or even kWh per hour! POWER is a rate. When we compare the POWER used by two appliances, we are comparing the rate at which they each use ENERGY. A rate often involves a period of time.

5. Other familiar rates: Dollars per hour (or “wages”) Miles per hour (driving speed) Gallons per minute (water flow) Meters per second (wind speed) Or, ENERGY rates can be, Joules per second kWh per month Both of these are measures of ENERGY usage, or POWER

6. What units are used to measure POWER? WATTS = Joules per second (1 W = 1 joule per sec) 1,000 watts = 1 kilowatt 1,000 kilowatt = 1 megawatt 1,000 megawatts = 1 gigawatt HORSEPOWER = (~746 Watts)

7. EXAMPLE: TWO calculations lead to the same result : One 25-watt light bulb burning for 1 hour uses 25 watts x 1 hour = 25 Wh (or 25 W-h) (.025 kW x 1 hour =.025 kWh (or.025 kW-h ) 25 joules x 1 hr = 25 joules x 3600 sec = 90,000 joules sec sec 90,000 joules x 0.278 kWh* = 0.25 kWh joule ____________________________________________________________________ A. B. *1 joule = 0.278 kWh

8. Analogies: a. Bacon has 50 Calories (per 0.1 g) b. Our car uses only 1 gal of gas (per 24 miles) c. My flat screen TV uses 1 kWh (per 15 minutes) d. My flat screen TV uses 550 watts e. The wind turbine can produce 1.5 mWh (in 3 days during the winter) f. The wind turbine can produce 1.5 mW g. Residences in Maine used a total of 4.360 GWh (in 2009)

9. Why this can be hard to teach: 1.ENERGY and POWER are often confused (or thought to be the same thing). 1.A watt (a measure of POWER) is a rate, but it doesn’t have a ‘per time’ in its name Watts = joules per second 3.RATES -- involve a fraction (division, ratio, proportion) -- involves two measures that can change, e.g. distance &time, energy &time, gallons &time 4. We often learn about watts first, then kWh. But really, kWh (a simple measure of ENERGY) is the simpler concept.