Since both the J and the kJ are
such small units, they are impractical when it comes to calculating the amount
of energy consumed by the average household. So instead we use the kWh ( kilowatt- hour).
The
kWh is obtained from the formula, E = P t.
If power. P, is in kilowatts(kW) and time,t, is in hours(h), we will obtain kWh.
The cost attributed to an appliance simply equals the energy it consumes, in kWh, times the electric company’s rate in $ / kWh.
Cost = E * rate
Use the table below for the examples that follow.
|
Appliance |
Power
Rating (W) |
Appliance |
Power
Rating(W) |
|
small
desk radio |
5 |
Microwave
oven |
1200 |
|
incandescent
light bulb |
60 |
washer |
1200 |
|
20
inch colour TV |
84 |
Water
heater |
4500 |
|
computer
+ screen |
168 +
240 = 408 |
dryer |
6480 |
|
2
door fridge |
780 |
Electric
furnace |
20
000 |
Example 1 While doing
your homework you leave the TV on for 24 hours.
If the rate is $0.05/kWh, how much will your action add to
your hydro bill?
E = 84W/1000(W/kW)* 24 h
= 2.016 kWh
Cost = E* rate = 2.016 kWh
*$0.05/kWh = $0.10
Example 2 You leave the window open on a very cold
day, and your furnace works for 9 hours. It continues to do so for the entire
week. How much does this add to your hydro bill?
E = 20000W/1000(W/kW)* 9 h/day * 7 days = 1260 kWh
Cost = E* rate =1260 kWh *$0.05/kWh = $63