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Electricity in the home: 1.4 Power ratings of appliances in the home and rate of energy transfer

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How much electrical energy is used by an appliance in the home?

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1.4 Power ratings of appliances in the home and rate of energy transfer

On the underside of this toaster is the label with the 'electrical' technical details.

You are informed the toaster works a power supply of 220-240 V AC at a frequency of 50 or 60 Hz.

AC means alternating current.

Power rating is a measure of the rate of energy transfer.

The power rating of the toaster is 1900 to 2300 W, depending on the voltage (p.d. across the heating element). The power is similar for all frequencies of 50-60 Hz.

This means the heating element is transferring energy at the rate of 1900 to 2300 J/s

From the information you can work out the current flowing through the heating element.

From: P = I x V, I = P / V. e.g. for a p.d. of 230 V and a power rating of 2100 W:

Current I = 2100 / 230 = 8.2 A (2 sf)

This appliance would be protected with a 10 A or 13 A fuse. For more details see calculate a safe fuse rating.

Examples of power ratings of things you find in the home - listed from the least powerful to the most powerful.

Appliance/machine Power rating W (J/s)
TV monitor   25
light bulb    50
small LED TV    85
refrigerator 100
food blender   160
microwave cooker 600
electric kettle 1200
dishwasher 1200
vacuum cleaner 1400
microwave cooker 1600
hairdryer 1800
steam iron 2000
hot water immersion heater 3000

Be careful NOT to equate power with the cost of using an appliance.

Time is the other factor, the longer you use an appliance, the more its use costs.

Some higher power appliances like the microwave or iron are only used for short times.

Computers, light bulbs and TV screens, might be on for many hours and the cost mounts up as more energy is transferred/work done!

The general word equation is: energy used = power x time

(see later section on kilowatt-hour calculations)

Most appliances are labelled with a power rating, which is the maximum power output with which it can be used safely.

The power rating tells you the maximum amount of energy transferred from one energy store to another per second when the appliances is being used.

e.g. a 700 W iron means 700 J of energy are being transferred (used) every second.

A 3 kW heater transfers to the thermal energy store of a room at the rate of 3000 J/second.

The power rating is useful information for the consumer.

The lower the power rating, the less electricity it uses, saving money - cheaper to run - as long as the appliance can still do what you want it to do.

e.g. if 500 W iron can do the work in the same time as a 750 W iron, then the 500 W iron is the more efficient and cheaper way to do your ironing!

750 - 500 = 250, so 250 J/s is saved to the thermal energy store of the clothes being ironed.

Whatever the power rating, its the efficiency of the appliance that is really important - what percentage of energy input is transferred in doing useful work.

However, beware!, just because an appliance has a higher power rating, it doesn't mean it is more efficient than a lower power appliance.

A higher powered appliance might waste more energy i.e. has lower % efficiency in terms of the electrical energy doing useful work.

Reminder: Power of appliance = current x potential difference

P (W or J/s) = I (A) x V (V)

For more questions see electrical power calculations section.

Apart from electrical power calculations for electrical appliances, this formula is needed to calculate a safe fuse rating.

INDEX of ELECTRICITY Notes 1. Electricity in the home

and also see

Conservation of energy, energy transfers-conversions, efficiency, calculations, Sankey diagrams

and for more on power calculations see

Types of energy & stores, calculations of mechanical work done and power


Keywords, phrases and learning objectives for appliance power rating of an appliance

Appreciate the different power ratings of appliances in the home and their relative rate of energy transfer.

Know that power in watts is the rate of energy transfer in joules per second.

Know how to use the power = current x p.d. equation e.g. how to calculate the current flowing knowing the power rating and mains voltage.


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