Section 3.3 Measuring
Energy Input and Output
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p. 332 - 338
We use energy in every aspect of our life.
Driving, ipods, heating our homes, cooking
food, watching tv.
How can we choose which type of energy
to use? (ex: we could use electricity,
gasoline, propane, hydrogen)

We need to measure the types and
amounts of energy going into and coming
out of the devices.
Power
Power is the rate at which a device
converts energy.
Unit of power is the watt (W).
1 watt is equal to 1 joule per second
1W = 1J/s

Ex: 44 W = 44 J/s
The faster a device converts energy, the
greater its power rating.
Is an appliance with high watts or low
watts better? Why?
Is an appliance with high watts or low
watts better? Why?

Depends on the way you look at it.
High watts, is more powerful, stronger, might not
have to use it as long
However, low watts is more energy efficient. Might
save you money in the long run.
Power formula
For an electrical device, the power is the
current multiplied by the voltage.
P = I x V (watts = amps x volts)
P
I
V
Power Calculation Example
A hair dryer has a power rating of 1000 W. It
is plugged into a 120-V outlet. What is the
current flowing through the hair dryer?
Known: Power (P) = 1000 W, Voltage (V) = 120 V
Unknown: Current (I) = ?
Formula: I = P/V
Solve: I = 1000 W/ 120 V
I = 8.33A
Energy
You can use the power rating of a device
to determine the amount of energy the
device uses (power is the rate at which a
device converts energy).
Energy formula
Energy consumption of an electrical
device is its input power multiplied by
the time the device is used.
E
P t
E = P x t (joules = watts x seconds)
Energy Calculation Example
A microwave oven has a power rating of 800W. If
you cook a roast in the oven for 30 min at high,
how many joules of electrical energy are converted
into heat by the microwave?
*Remember that W = J/s
• Known: P = 800 W (J/s), t = 30 min
30 min = ? s
30min x 60 s/min = 1800s
• Unknown: E = ?
• Formula: E = P x t
• Solve: E = 800 W x 1800s
E = 1 440 000 J = 1.4 MJ
Write this at the bottom of the
page…
Kilowatt Hours
It doesn’t take common electrical devices
long to consume a large number of
joules.
Because of this, the kilowatt hour is often
used.
Energy calculation is the same, except you
use hours instead of seconds, and
kilowatts instead of watts.
1 kW = 1000 W