Practice Energy Calculations
Useful Information:
• Formula to find electrical energy is: Energy = power x time
• Duke Power charges about 8¢ per kWh.
• 1 ton of coal produces an average of 2500 kWh of electricity.
• Each kWh of electricity generated from coal produces 1.5 lbs of CO2.
• 1 gallon of gasoline produces about 22 lbs of CO2 when burned.
(1) Assume that the lights in your kitchen use 300 watts. How much energy and how much does it cost
to leave the lights on 24 hours a day for a week if electricity is 8cents/kilowatt hour?
For a month (assume 30 days/month)?
For a year?
(2) How much energy and how much money do you use to run your window air conditioner rated at 1500
watts continuously for the month of July (assume 8¢/kWh).
If you assume that coal was used to produce the electricity for your air conditioner, how much coal
was burned to produce the electricity used?
How much CO2 was produced by the electricity used to run your air conditioner?
(3) An average incandescent light bulb has a life expectancy of 1,000 hours. How much energy would a
typical 60 watt bulb use in a lifetime, assuming it lasts for 1,000 hours?
At 8¢/kWh, how much would it cost over its lifetime?
(4) A compact fluorescent bulb uses 15 watts and has a life expectancy of 10,000 hours. How much
energy and how much would it cost to use a compact fluorescent for 10,000 hours?
(5) If your car gets 20 miles per gallon (MPG), and you drive an average of 10,000 miles each year, how
many gallons of gas do you use a year?
At $2.00 per gallon, how much will you spend on gasoline for the year?
If the combustion of each gallon of gasoline produces 22 lbs of CO2, how much CO2 does your car
produce each year?
If you traded your car in & bought one that got 25 MPG, how much gasoline would you save in one
year?
How much money would you save?
How much less CO2 would be emitted into the atmosphere from your improved car?
AP Environmental Science
KEY
Practice Energy Calculations
Useful Information:
•
Formula to find electrical energy is: Energy = power x time
•
Duke Power charges about 8¢ per kWh.
•
1 ton of coal produces an average of 2500 kWh of electricity.
•
Each kWh of electricity generated from coal produces 1.5 lbs of CO2.
•
1 gallon of gasoline produces about 22 lbs of CO2 when burned.
(1) Assume that the lights in your kitchen use 300 watts. How much energy and how much does it cost to
leave the lights on 24 hours a day for a week if electricity is 8cents/kilowatt hour?
energy = power x time
50.4 kWh $0.08
æ24 hrs 7 days ÷
ö
æ 1 kW ÷
ö
cos t =
x
ç
ç
÷
300 W ç
= .3 kW 
= .3 kW ç
x

wk
kWh
÷
÷
çè1000 W ÷
÷
ç
ø
wk ø
è day
= $4.03 / wk
= 50.4 kWh
wk
For a month (assume 30 days/month)?
energy = power x time
216 kWh $0.08
æ24 hrs 30 days ÷
ö
cos t =
x
ç
÷
= .3 kW ç
x

mo
.
kWh
÷
çè day
mo. ÷
ø
= $17.28 / mo.
kWh
= 216
mo.
For a year?
energy = power x time
2628 kWh $0.08
æ24 hrs 365 days ÷
ö
cos t =
x
ç
yr
kWh
÷
= .3 kW ç
x

÷
yr ÷
èç day
ø
= $210.24 / yr
kWh
= 2,628
yr
(2) How much energy and how much money do you use to run your window air conditioner rated at 1500
watts continuously for the month of July (assume 8¢/kWh).
energy = power x time
1116 kWh $0.08
æ24 hrs 31 days ÷
ö
æ 1 kW ÷
ö
cos t =
x
ç
ç
1500 W ç
= 1.5 kW 
÷
= 1.5 kW ç
x

mo.
kWh
÷
÷
çè1000 W ÷
ç
÷
ø
mo. ø
è day
= $89.28 / mo.
kWh
= 1,116
mo.
If you assume that coal was used to produce the electricity for your air conditioner, how much coal
was burned to produce the electricity used?
1 ton = 2,000 lbs.
2,000 lbs. of coal = 2500 kWh
1116 kWh
2000 lbs.
x
= 892.8 lbs. of coal / mo.
mo.
2500 kWh
How much CO2 was produced by the electricity used to run your air conditioner?
1116 x 1.5 lbs. CO 2 = 1, 674 lbs. CO 2
AP Environmental Science
Pg. 1
KEY
(3) An average incandescent light bulb has a life expectancy of 1,000 hours. How much energy would a
typical 60 watt bulb use in a lifetime, assuming it lasts for 1,000 hours?
æ 1 kW ÷
ö
60 W çç
= 0.06 kW 
÷
çè1000 W ÷
ø
energy = power x time
= 0.06 kW x 1, 000 hrs.
= 60 kWh
At 8¢/kWh, how much would it cost over its lifetime?
60 kWh x $0.08 / kWh = $4.80
(4) A compact fluorescent bulb uses 15 watts and has a life expectancy of 10,000 hours. How much
energy and how much would it cost to use a compact fluorescent for 10,000 hours?
æ 1 kW ÷
ö
15 W çç
= 0.015 kW 
÷
÷
çè1000 W ø
energy = power x time
= 0.015 kW x 10, 000 hrs.  150 kWh x $0.08 / kWh = $12.00
= 150 kWh
(5) If your car gets 20 miles per gallon (MPG), and you drive an average of 10,000 miles each year, how
many gallons of gas do you use a year?
10, 000 mi / yr
= 500 gal / yr
20 mi / gal
At $2.00 per gallon, how much will you spend on gasoline for the year?
500 gal
$2.00
x
= $1, 000 / yr
yr
gal
If the combustion of each gallon of gasoline produces 22 lbs of CO2, how much CO2 does your car
produce each year?
500 gal
22 lbs of CO 2
x
= 11, 000 lbs CO 2 / yr
yr
gal
If you traded your car in & bought one that got 25 MPG, how much gasoline would you save in one year?
10, 000 mi / yr
= 400 gal / yr
25 mi / gal
100 gallons of gas/yr
How much money would you save?
400 gal
$2.00
x
= $800 / yr
yr
gal
$200 per year
How much less CO2 would be emitted into the atmosphere from your improved car?
400 gal
22 lbs of CO 2
x
= 8,800 lbs CO 2 / yr
yr
gal
AP Environmental Science
2,200 lbs CO2/yr
Pg. 2