Electricity cost in our homes

Most appliances we use in our homes have labels giving their power ratings. Power ratings are given in watts or kilowatts and indicate the rate at which an appliance uses electrical energy. In other words, the amount of electrical energy the appliance uses per second.

The unit of power, the watt (w), is equal to a joule per second.

Power = energy/time

1w = 1 j/s

A power rating of 1w means that the appliance uses 1 joule of electrical energy per second.

Therefore, a power rating of 1000w means that the appliance uses 1000 joules of electrical energy per second.

The table below provides typical appliances power ratings at 220 V ac.

 Appliance Power rating (kW) Stove (total) 14 Hot water cylinder 4 Washing machine 2.75 iron 1 Hair drier 1 1 bar heater 1 Vacuum cleaner 0.8 toaster 0.9 Micro wave 0.65 refrigerator 0.11 Light bulb 0.06 Radio cassette 0.012

We can the formula:

Power = voltage x current

To calculate the current drawn by an appliance if we know the voltage at which the appliance operates at 220 V to 240 V ac.

What current does a 2kw kettle operating at 240V ac draw?

I = P/V

I = 2000W/24

I = 8.7 A

We can now use Ohm's law to calculate the resistance of the kettle.

voltage = current x resistance (v = IR)

Therefore, the kettle has a resistance of

R = V/I

R =240/8.7A

R = 26 OHMS

Note the power rating of the kettle in watts (W)

Comparing two kettles with different power rating

Consider two kettles operating at 240V ac. One with a power rating of 1000 W and one with a power rating of 2000 W.

Which two kettles will boil equal quantities of water faster?

Why?

In order to calculate how much electrical energy a kettle uses we need to multiply the kettle's power rating by the time taken to reach boiling.

ENERGY = POWER X TIME

Consider the 1000 W kettle.

It takes 4 minutes to boil. The electrical energy used will be :

ENERGY = 1000 W X 240 s

= 240000 J

=240 kJ

Consider the 2000 W kettle

This kettle takes 2 minutes to boil. The electrical energy used will be:

ENERGY = 2000 W X 120 s

= 240000 J

=240 kJ

Therefore the kettle uses the same amount of electrical energy.

KILOWATT- HOURS: A UNIT OF ELECTRICAL ENERGY

As you can see from our calculations above, to boil a kettle requires many thousands of joules.

For electricity suppliers such as ESKOM to work with such large numbers would be inconvenient, instead electrical energy is measured in units called kilowatt- hours (kWh).

How many kWh are used to bring the 2000 W kettle to the boil?

Take care to check your units of energy if they are correct.

Firstly, express the power rating in kW.

Power rating in kilowatts = kW

Secondly, express the time taken to boil in hours.

Time taken to boil in hours = hr

Thirdly, calculate the time taken for water to boil in hours.

energy used in kWh = kilowatts x hours

= kWh

Calculating the efficiently of a kettle

The efficiency of a kettle

Energy efficiently is the ratio of how much energy goes in to an appliance to the amount of useful energy got out. A kettle is not a perfect insulator, some of the electrical energy supplied to the kettle goes into heating the kettle and the surrounding air. Also heat is "lost" through water that vaporises before the kettle boils. The more energy "lost" the lower the efficiency.

Efficiency is given by the ratio of the electrical energy supplied to the kettle to the energy actually needed boil the water. Efficiency is usually expressed as a percentage the greater the efficiency.

REDUCING ELECTRICITY COSTS IN OUR HOMES

ESKOM supply local authorities, municipalities and councils with electricity and they in turn supply users living within their areas.

Local authorities can charge their own rates for the electricity they supply.

A LOOK AT SOME DOMESTIC APPLIANCES

In order to be able to conserve electricity we need to know which appliances use the most electrical energy.

This means considering both appliance power ratings and the time for which they are used.

The two most important applications of domestic electricity are heating and lighting. Heating usually requires much more electrical energy than lighting.

Each of the important appliances will now be discussed.

Hot water Cylinders

Most hot water cylinders keep a large volume of water heated at a selected temperature for 24 hours a day. Because of the convenience of having the water permanently heated, hot water cylinders use the most electrical energy in the home.

The device that sets the temperature is called a thermostat.

The heating element in a hot water cylinder is similar to that of a kettle. When the water temperature drops below the selected temperature the hot water cylinder automatically switches on. The better insulated the cylinder is, the longer it takes for it takes for the water to cool down, and element will be switched on less often.

How to reduce the amount of electrical energy your hot water cylinder mass uses.

You can check the thermostat setting. It might be possible to reduce the setting and still have hot water at a comfortable temperature. In summer it might be possible to reduce the setting further. Other hot water measures include showering instead of bathing and making sure you don't use the hot tap for cold water.

Electric Stoves

Electric stoves are heavy users of electricity.

Some things you could do to reduce your monthly electricity bill:

1) You must make sure that the pots and pans you use have flat bottoms.

This will improve the conductivity between the hot plate and the pot or pan.

2) Match the size of the pots and pans with the pot or pan.

3) Keep the lids on, to avoid wasting heat.

4) Do not use more heat than you need to cook food.

5) Use a kettle for boiling water- it's more efficient.

Electric Heaters

The power ratings of electric heaters depend on the type of heater. Most heaters have a power rating of 1 kW or greater.

Some things you could do to reduce your monthly electricity bill:

1) ONLY use heaters when it is necessary.

2) CLOSE the windows or doors of the room you are heating.

3) TURN the heater off when the temperature is comfortable.

Note: If we compare the cost of operating a 1kW heater and a 60W bulb for one hour we find that the heater costs nearly seventeen times more. Can you prove this?

Refrigerators

Like hot water cylinders, refrigerators are thermostatically controlled. This means the refrigerator goes above the set temperature.

Some things you could do to reduce your monthly electricity bill:

1) Do not place hot foods in the refrigerator. Let them reach the room temperature first.

2) Make sure that the refrigerator door seals closed properly

3) Do not open the refrigerator door unnecessary, and do not leave it open.

A take home exercise

List all the electrical appliances in your home in order of their power ratings.

Which contribute the most to the electric bill?

Remember that we are charged for the electrical energy we use. This iis given by the two factors:

-The appliance power rating,

-Time which the appliance was used.