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OBJECTIVE

To practice calculating energy usage and costs.

MATERIALS

❖ Calculator

BACKGROUND

The basic unit of electrical power used for measuring work is the watt (W).

A megawatt (MW) is the amount of work or energy equal to using 1,000 kilowatts or 1 million watts.

A kilowatt hour (kWh) is the amount of work or energy equal to using 1,000 watts for one hour. Your electric bill is computed according to the number of kWhs that you use.

PROCEDURE

Show your work on the following problems. Be sure to include the correct units in your answers. Round all answers to two decimal places:

The basic formula to find kilowatt hour use is: kWh use = watts x hours / 1000.

Example: How many kWhs are used by a 100-watt light bulb left on for 18 hours?

kWh use = 100 watts x 18 hours = 1.8 kWh

1000

1. How many kilowatt hours are used per day on a 75-watt electric blanket for 7 hours per night?

2. How many kilowatt hours are used by a 1450-watt microwave oven in use for 30 minutes?

Electricity costs are usually based on kilowatts/hour charges. To find the cost of using an appliance on a monthly basis, use this formula:

days used x hours used/day x kW x average rate of electricity = cost per month

Example: What is the monthly cost to watch a 230-watt color TV for 5 hours per day? ***Assume electricity costs $0.13 per kWh for all problems***

***Make sure to check your answer units (i.e. watts should be in kW)***

30 days x 5 hrs/day x .230 kW x 0.13 = $4.48 per month

3. What is the monthly cost to leave a 100-watt light bulb on for 18 hours per day?

4. What is the monthly cost of running a refrigerator that uses 3300 watts/day?

(hint: refrigerators do not run for 24 hours/day)

5. Which is less expensive and by how much: cooking a baked potato in a 1450-watt microwave oven for 7 minutes or cooking with a conventional oven for one hour at 600 watts per hour use? (Hint: It’s only for one day)

Background Info: 1 BTU = amount of energy needed to raise the temperature of 1 lb of water by 1º F.

1 gallon of water = 8 lbs 1 kW = 3400 BTUs

Example: What is the cost to heat 20 gallons of water by 50º F?

# of gallons x 8 lbs/gal = # of lbs x # of degrees changed = BTU / 3400 = kW

20 gallons x 8 lbs = 160 lbs x 50º = 8000 BTU / 3400 = 2.35 kW x $0.13 = $0.30

6. Compare the cost of a 10-minute shower that produces 3 gallons of water per minute with a bath in a tub that holds 40 gallons of water. The water temperature for both will be 100º F, but the water is at 50º F when it comes into the house. Which is less expensive and by how much?

CONCLUSION

Type your answers and attach them to this packet.

1. Define energy.

2. What are some forms of energy?

3. What is the term for energy of motion and give an example?

4. Is the heat energy of the ocean high or low quality? Why isn’t it used as an energy

resource?

5. Name four examples of renewable and four examples of non-renewable sources of

energy.

6. What are four types of electricity sources used in Jacksonville? Which is the most

common?

7. What are some various methods that energy can be conserved (at least five ways)?

8. What is JEA’s residential electric rate per kWh?

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LAB #11 – ENERGY CALCULATIONS

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