OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
(c) the kilowatt-hour (kW h) as a unit of energy; calculating the cost of…
Full specification wording
Power: (c) the kilowatt-hour (kW h) as a unit of energy; calculating the cost of energy.
The kilowatt-hour is a practical unit of energy: one kW h is the energy transferred by a 1 kW device operating for 1 hour.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 6 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The kilowatt-hour is a practical unit of energy: one kW h is the energy transferred by a 1 kW device operating for 1 hour. Since 1 kW = 1000 W and 1 h = 3600 s, 1 kW h = 1000 × 3600 = 3.6 × 10⁶ J. To find energy in kW h, multiply power in kilowatts by time in hours. For a 2.0 kW heater used for 3.0 hours, energy = 2.0 × 3.0 = 6.0 kW h. If electricity costs 30 p per kW h, total cost = 6.0 × 30 = 180 p, or £1.80. In multiple-choice questions, check whether power is given in watts or kilowatts and whether time is in hours or seconds before calculating.
What earns the marks
- Defines the kilowatt-hour as the energy transferred by a one-kilowatt device in one hour.
- Converts between kW h and joules using 1 kW h = 3.6 × 10⁶ J.
- Calculates energy in kW h by multiplying power in kW by time in hours.
- Calculates cost by multiplying energy in kW h by the unit price.
- Handles unit conversions between watts and kilowatts and between minutes and hours.
Common mistakes and how to put them right
Treating kW h as a unit of power.
Put it right: kW h measures energy, while kW measures power.
Using power in watts directly with time in hours.
Put it right: Convert watts to kilowatts by dividing by 1000 first.
Forgetting to convert pence to pounds when the question asks for cost in pounds.
Put it right: Divide the pence total by 100.
Exam tips
- Convert power to kilowatts and time to hours before multiplying to obtain kW h.
- Keep the unit price consistent with the energy unit used in the calculation.
- Check whether the answer should be in pence or pounds and convert if necessary.
Try the lesson
The start of the Power: (c) the kilowatt-hour (kW h) as a unit of energy; calculating the cost of energy. lesson, made from checked teaching notes for this specification point.
Listen
The kilowatt-hour is a practical unit of energy: one kW h is the energy transferred by a 1 kW device operating for 1 hour. Since 1 kW = 1000 W and 1 h = 3600 s, 1 kW h = 1000 × 3600 = 3.6 × 10⁶ J. To find energy in kW h, multiply power in kilowatts by time in hours. For a 2.0 kW heater used for 3.0 hours, energy = 2.0 × 3.0 = 6.0 kW h. If electricity costs 30 p per kW h, total cost = 6.0 × 30 = 180 p, or £1.80. In multiple-choice questions, check whether power is given in watts or kilowatts and whether time is in hours or seconds before calculating. A strong answer earns marks for these points. Defines the kilowatt-hour as the energy transferred by a one-kilowatt device in one hour. Converts between kW h and joules using 1 kW h = 3.6 × 10⁶ J. Calculates energy in kW h by multiplying power in kW by time in hours. Calculates cost by multiplying energy in kW h by the unit price. A common slip: Treating kW h as a unit of power. Instead, kW h measures energy, while kW measures power. Exam tip: Convert power to kilowatts and time to hours before multiplying to obtain kW h.
See it
Key words
Say what each word means before you turn the card.
kilowatt-hour
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conversion between kW h and joules
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energy in kW h
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cost calculation
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See every definition
- kilowatt-hour
- A unit of energy equal to the energy transferred by a 1 kW device operating for 1 hour.
- conversion between kW h and joules
- Since 1 kW = 1000 W and 1 h = 3600 s, 1 kW h = 1000 × 3600 = 3.6 × 10⁶ J.
- energy in kW h
- Found by multiplying power in kilowatts by time in hours.
- cost calculation
- Total cost is found by multiplying energy in kW h by the unit price.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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