Kilowatt-Hours (Oxford AQA IGCSE Physics)
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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