Define a kilowatt-hour and convert between energy units
Interpret kWh as an energy amount, convert it to joules and calculate appliance energy from kW and hours.
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Distinguish energy from power.
- Convert kilowatts and hours.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How many seconds are in one hour?
What you will learn
- Define one kilowatt-hour as energy transferred at one kilowatt for one hour.
- Convert kWh and joules while keeping power and energy distinct.
Multiply a power by a duration
One kilowatt-hour is the energy transferred at a constant rate of one kilowatt for one hour. Since 1 kW = 1000 W and 1 h = 3600 s, 1 kWh = 3 600 000 J. The unit is a product of power and time; it is not 'kilowatts per hour', which would describe a different kind of rate.
For an appliance with a stated constant electrical input, energy in kWh equals power in kW multiplied by time in hours. A 2 kW appliance operating for 0.5 h uses 1 kWh. A 500 W appliance for 2 h also uses 1 kWh after converting 500 W to 0.5 kW.
Keep the units attached
For a joule calculation, use watts and seconds. For a kWh calculation, use kilowatts and hours. Do not multiply a value in watts by hours and label the result kWh without converting: 500 W × 2 h is 1000 Wh, equal to 1 kWh.
A device rated 2 kW has an input rate at its specified condition, not a fixed lifetime energy use of 2 kWh. Its actual accumulated energy depends on duration and control cycles. Use measured meter energy or stated average power if the appliance cycles on and off.
Sources for this lesson
Pause and explain
How much energy does 2 kW use over 0.5 h at constant input?
Worked example
Define 1 kWh and convert it to joules.
Show the worked solution
- It is 1 kW operating for 1 h.
- Convert to 1000 W and 3600 s.
- Energy = 1000 × 3600 = 3 600 000 J.
Answer 1 kWh = 3 600 000 J.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Convert 1 kWh to joules.
Give me a hint
Change both parts of the product into watts and seconds.
Compare my reasoning
- 1 kW = 1000 W.
- 1 h = 3600 s.
- 1 kWh = 3 600 000 J.
3 600 000 J.
Look for these in your work
- Convert both units.
- State energy in joules.
Find energy in kWh and joules for a steady 500 W input over 2 h.
Give me a hint
Use kW and hours first, then the kWh-to-joule conversion.
Compare my reasoning
- 500 W = 0.5 kW.
- Energy = 0.5 × 2 = 1 kWh.
- Energy = 3 600 000 J.
1 kWh; 3 600 000 J.
Look for these in your work
- Convert watts to kilowatts.
- Retain the duration.
A label states 2 kW. Can its daily energy use be read as 2 kWh without any timing or control information?
Give me a hint
A power rating lacks the day's effective operating duration.
Compare my reasoning
- 2 kW is a rate at a specified condition.
- Daily energy depends on operating time and control cycling.
- No unique daily kWh amount is established.
No; duration or measured daily energy is needed.
Look for these in your work
- Separate rate and amount.
- Identify missing usage data.
Common mistakes
- kWh is an energy unit.
- kWh is not kW/h.
- A rated power does not supply operating duration.
What can you explain now?
Which combination directly gives kWh?
Compare with the explanation
Kilowatts multiplied by hours.
The matching power and duration units make the accumulated amount a kilowatt-hour value.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Define kWh, convert to joules and select power–time units that give the requested energy unit.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.