1. Power

2. Energy The ability to do work. Potential Energy = stored energy
Kinetic Energy = energy in motion.
The different types of energy can be converted into each other.
Ex. Electrical  Light

3. Power Is the rate of energy transformation
Amount of energy used in a certain amount of time
P = E/t
Measures in units called Watts (W)
1 Watt = 1 J/s
Power is also related to current and voltage!
How much voltage and current will your appliances use?

4. Calculating Power We can use the formula: Power = Current * Voltage
𝑃=𝐼×𝑉
P = Power (Watts)
V = Voltage (Volts)
I = Current
(Amps NOT MilliAmps)

5. Practice
A solar cell produces 1.8 A of current at 0.42 V. What is the power of the solar cell?
P =
I =
V =

6. Practice
2. A 37 W cordless drill uses a current of 2.6 A. What is the voltage of the battery? P = I = V =

7. Household Appliances
Recall that house hold outlets supply 120 V (220 V for stove and dryer)
Most appliances are labelled with the amount of power (Watts) they consume
Using this information, the current can be calculated.

8. Practice
3. What is the current in a 95 W television set that is connected to a 120 V household outlet? P = I = V =

9. Why do some appliances use 220V?
A clothes dryer needs a lot of power to run, approx W (7.2 kW)
What is the current needed if 120 V outlet is used?
7200/120= 60 A
What is the current needed if 240 V is used?
7200/220=? A
60 A represents 2/3 of an entire house’s current (100A). By using a higher voltage, it reduces the current needed & allows for smaller wires to carry the current

10. Assignment
11. 2 (p. 344) #2, 4, 5, 6, 9, 10

11. Energy

12. Electrical Energy Power = Energy/Time, so the formula rearranged is:
Energy = Power x Time
Power – change to KiloWatts
Time in hours
(3600 sec = 1 hr)
Energy Units: kW•h

13. Cost of Energy We are charged by the amount of energy we consume.
BC hydro charges by the amount of Kilowatts consumed over a period of time
(1kW = 1000W) •Hours (3600 sec = 1 hr)
Energy Units: kW•h
Cost of Electricity = Energy (kWh) x rate ($/kWh)

14. Practice Unit Conversion
1500 W = ______ kW
500 W = _______ kW
100 W = _______ kW
1200 W = _______ kW
10 min = ________ h
1 day = _________ h
1 week = _________ h
30 min = _________ h

15. Sample Problem #1 You use a 1100 W microwave for 30 minutes each day.
1. How much energy (kW • h) do you use in one week?
E = P (kW) x time (h)
P = 1100 W = 1.1 kW
t = 30 minutes = 0.5 h/day * 7 days/wk = 3.5 hrs/week
E= 1.1 kW x 3.5 h = 3.85 kW • h
2. If electricity costs $0.07/kW•h, how much does it cost to run your microwave for one week?
Cost = Energy x rate = 3.85 kW•h x $0.07 = $0.27

16. Sample Problem #2
A 9.6 Ω kettle is plugged into an outlet. What will it cost to operate the kettle for 20 minutes, if electricity costs $0.065/kW•h?
List the information you are given.
R= 9.6 Ω
V = 120V (regular household outlet)
t = 20 min = 20/60 min = 1/3 h ( h)
We need to find the power!! But P = IV, and we only have V 
We can use ohm’s law to find current, and then use current and voltage to find power 

17. Sample Problem #2 continued….
I = V/R
120V/9.6 Ω = 12.5A
P = IV
12.5A * 120V = 1500W
Convert W to kW = 1.5kW
E = P * t
1.5 kW * 1/3 h = 0.5 kW •h
Cost = E x rate
0.5 kW •h * $0.065 =
$0.03

18. Electricity Costs What does BC hydro charge for electricity?
Basic charge _____________ /day (paying for the right to access the electricity )
Usage charge _____________/ kWh
If users exceed the “threshold” (732 kWh) in a month, they pay more electricity: $ /kWh

19. Calculating your monthly electricity bill

20. Energuide Labels Appliances have labels that indicate the amount
of energy consumed
Allow for comparison
between two appliances
as to the best energy
efficient one.

21. Reading a Label:
1. Average annual energy consumption of the appliance in kilowatt hours (kWh)
2. Energy efficiency of the appliance
relative to similar models
3. Annual energy consumption range
for models of this type and size
4. Type and size of the model
5. Model number

22. Choose the appliance with the lowest annual kW.h rating
The energy-wise and environmentally conscious choice may also be the most economical, as the most energy-efficient washer will cost you $ less than Model X to operate over the life of the appliance. And remember, when the cost of electricity rises, so will your savings
Annual electricity cost calculation:
136kWh × $ kWh × =$ = $ × x life expectancy life expectancy (14 years) (14 years) = $ = $814.80

23. Assignment
11.4 (p. 354) #3, 5, 8, 10, 12
Electrical Energy Problems Worksheet