How much energy do things use?
How much work are thing doing
• Before we can understand how much it costs
to run an appliance we must first calculate
how much power it requires to run.
POWER!!!!
• To find the power of an appliance you
complete the following equation
• Power = Voltage x Current
(Watts W) (Volts V)
(Amps A)
Normally this information can be found on the
ratings plate of an appliance
What is the power rating of this?
• Power = Voltage x Current
(Watts W) (Volts V)
(Amps A)
Working out the cost of running this
appliance
• We need to know
several things to
calculate this.
1) How energy
transfer is
measured
2) The cost of a unit
of electricity
How energy is measured.
• Energy is measured in joules.
• An appliance with a rating of 1watt uses 1 joule
of energy every second.
• Most household appliances use a lot of energy so
we measure the energy use in kilowatts, this is
equal to 1000 watts.
• Summary
• 1 joule = 1 watt
• 1 kilowatt – 1000 watts
Complete part 1 on the worksheet
• How well are you doing?
The kilowatt hour
• We measure the energy used by household
appliances in kilowatt-hours (kWh) rather than
using energy per second
• The kilowatt hour (kWh)
= 1000W x (60 x 60s)
= 3600000J
The kilowatt hour (kWh)
= 1000W x (60 x 60s)
= 3600000J
Quite simply this is the amount of
energy used by a 1kW appliance left
on for an hour
Calculating energy of on object
• This equation
• Energy used = power rating x
(Joules j)
(watt W)
Kilowatt- hour (Kilowatt kW)
kWh
time
(seconds s)
(Hours h)
So how much energy does this use if it is
left on for 1.5 hours?
Energy used = power rating x time
(Joules j)
(watt W)
(seconds s)
Kilowatt- hour (Kilowatt kW)
(Hours h)
kWh
Complete part 2
• How do you feel?
• Feeling confident?
Answer the following question
What will this cost?
• If the cost is 12p per unit ( a unit is equal to 1 kWh)
• Cost of energy = Number of units used x price per unit
Pence p
kWh
price per kWh
Energy Efficiency
• Not all house hold appliances are energy
efficient
• Being energy efficient means that the
appliance is doing the job you want it to.
• For example a bulb
• Is this energy efficient?
To calculate the efficiency of an
appliance
• Calculating efficiency
• The efficiency of a device such as a lamp can be calculated using this
equation:
• efficiency = ( useful energy transferred ÷ energy supplied ) × 100
Complete the extension questions