Kilowatt-Hours (Oxford AQA IGCSE Combined Science Double Award)
Revision Note
Units of Energy
Energy measured in joules
Electrical energy transferred is often calculated with units of joules
One joule is equivalent to one-watt second
Consider an average lightbulb with a power of 60 W, which is left on for 6 hours in a house
1 hour is 3600 s
The electrical energy transferred over this time is 1.296 × 106 J
This number is large and that is one lightbulb for a single day
A household uses many appliances all year round – the energy transferred per month in joules would be inconveniently large
Energy measured in kilowatt-hours
To make these large values more relatable to daily use:
Power can be measured in kilowatts (kW)
Time can be measured in hours (h)
In this case, energy has units of kilowatt-hours (kW h)
The lightbulb from before receives 3.6 kW h of energy over the 6 hours
This value is much easier to understand for consumers and energy providers – thinking in terms of hours of use is more practical than seconds
Kilowatt-Hours
As has been stated previously, the equation for energy transferred is:
But here we consider different units:
E = energy transferred measured in kilowatt-hours (kW h)
P = power of the appliance measured in kilowatts (kW)
t = time measured in hours (h)
The usual unit of energy is joules (J), which is one watt-second
To find the number of joules in 1 kW h, convert the power and time to watts and seconds
1000 watts multiplied by 3600 seconds is equal to 1000 multiplied by 3600, in watt-seconds
Therefore, 1 kWh = 3.6 × 106 J
To convert from kW h to J:
To convert from J to kW h:
The kW h is a large unit of energy, and is mostly used for energy in homes, businesses and factories
Worked Example
A cooker transfers 1.2 × 109 J of electrical energy to heat.
Calculate the cost of this if 1 kW h costs 14.2 p.
Here, 100 p = £1 (100 pence = 1 pound).
Answer:
Step 1: List the known quantities
Energy in joules, E (J) = 1.2 × 109 J
Cost per kW h = 14.2 p
Step 2: Convert from J to kW h
Step 3: Calculate the price
Examiner Tip
The equation for energy in kilowatt-hours, with the units of each variable, is given to you on your equation sheet.
You can either memorise the conversions between J and kW h or convert manually, but ensure you are confident in this conversion.
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