P1aL11 Cost of Electricity

Key Words

Electrical Appliance - a device that terns electricity into useful energy

Electrical current - how many electrons flow in one second

Joule - unit of energy.

Kilowatt - 1000 watts or 1000 J/s.

Kilowatt hour - the energy used by a 1 kW appliance running for 1 hour.

Power rating - how much power an appliance uses

Series circuit - a circuit where the electric current passes all the components one after another

Watt - unit of power.  1 W = 1 J/s.

Grade E

Electric current is not used up.  The current in a series circuit is the same all the way round; the current going into a battery is the same as the current coming out.  The same is true of our homes.  The current is detected by the electricity meter which measures how much energy we use and have to pay for.

All appliances have a power rating, which is stuck to the back of the appliance.

This motor is rated at 1/3 horsepower which is 250 W or 0.25 kW

Power is measured in watts where 1 watt (W) = 1 joule per second.  With high powered appliances, the power is in kilowatts (kW), where 1 kW = 1000 W.   A 3 kW kettle transforms electrical energy into heat heat energy at a rate of 3000 J every second.

Grade C

The cost of electricity is worked out by working out how much energy is used:

energy transferred (J) = power (W) × time (s)

In Physics code:

E = Pt

In triangle form:

Example

A 2 kW kettle takes 3 minutes to boil.  What is its energy use?

       Convert minutes to seconds: 3 min = 180 s.  Convert 2 kW to watts:  2 kW = 2000 W

       Energy used = 2000 W × 180 s = 360 000 J

So even a small kitchen task takes a lot of joules.  Joules are really too small to use for working out our energy consumption for a quarter (3 months).  Instead we use a much bigger unit of energy called the kilowatt-hour (kWh).

kilowatt hours (kWh) = power (kW) × time (h)

Going back to our kettle above:

     Convert 3 minutes to hours: 3 min = 3/60 = 0.05 h.

     Energy used = 2 kW × 0.05 h = 0.1 kWh.

To work out the cost we need to know how much electrical energy costs per kilowatt hour.  Currently it's about 10 p.

cost (p) = kilowatt hours used (kWh) × cost per kilowatt hour (p/kWh)

So our kettle will cost 0.1 kWh × 10 p/kWh = 1 p

On your electricity bill you may see kilowatt-hours referred to as units.  1 unit = 1 kWh.

Grade A

To save energy it's necessary to turn appliances off instead of on standby wherever possible.  Some appliances which have clocks in them, like video recorders cannot be turned off.  A video recorder operating might take 15 W when running, but use 10 W on standby.  Although this may not sound a lot, a number of appliances around the home left on standby might well increase your electricity bill significantly. Across the whole country, it adds up to a significant amount of energy.