Calculate appliance energy costs from kWh tariffs
Use supplied fictional unit prices to calculate the energy part of an appliance's cost with clear billing boundaries.
Section P4.4 · Electrical energy
Cross-referenced from the related Cambridge IGCSE Chemistry 0620 or Physics 0625 teaching to the 0652 syllabus. Exact 0652 Core/Supplement objective coverage and this final-series course’s examiner fit still need specialist review. Written practice is self-checked and does not predict a Cambridge grade or demonstrate supervised laboratory competence.
Before you begin
- Calculate energy in kWh.
- Multiply decimal quantities with units.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
A steady 0.8 kW input runs for 1.5 h. What energy is used?
What you will learn
- Calculate kWh from stated appliance power and duration.
- Multiply energy by a supplied per-kWh price.
Calculate energy before price
For a supplied unit price, energy cost = energy in kWh × price per kWh. A constant 0.8 kW appliance running for 1.5 h uses 1.2 kWh. At a fictional price of 0.25 currency units per kWh, the energy charge is 0.30 currency units. These are practice prices, not current real electricity tariffs.
The price applies to energy, not to the power rating alone. Two appliances with different powers can have the same energy cost if their power–duration products match. Record the unit price and interval so the reader can see exactly which charge has been calculated.
Read the billing boundary
This calculation covers the specified energy charge. A total bill may also have standing charges, tax, tiered prices or other supplied terms; include them only if the question specifies them. If a fixed daily charge is supplied, add it once for that billing period rather than multiplying it by appliance energy.
Use actual or supplied average electrical input when an appliance cycles on and off. A 2 kW appliance that is on for one quarter of a 4 h interval has an effective on-time of 1 h, using 2 kWh under the stated zero-off-input model. Four hours of rated operation would be a different assumption.
Sources for this lesson
Pause and explain
At 1.2 kWh and 0.25 currency units/kWh, what is the energy cost?
Worked example
A constant 0.8 kW input runs for 1.5 h at a fictional 0.25 currency units/kWh. Find the energy charge.
Show the worked solution
- Energy = 0.8 × 1.5 = 1.2 kWh.
- Price = 0.25 currency units per kWh.
- Cost = 1.2 × 0.25 = 0.30 currency units.
Answer 1.2 kWh; energy charge 0.30 currency units.
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.
Find energy cost at 0.8 kW for 1.5 h using a fictional price of 0.25 currency units/kWh.
Give me a hint
Calculate the energy product before applying the unit price.
Compare my reasoning
- Energy = 1.2 kWh.
- Cost = 1.2 × 0.25.
- Energy charge = 0.30 currency units.
1.2 kWh; 0.30 currency units.
Look for these in your work
- Show the energy amount.
- Keep the charge boundary clear.
A 2 kW appliance is on for one quarter of a 4 h interval and uses zero input while off. At 0.30 currency units/kWh, find its energy cost.
Give me a hint
Use effective on-time under the explicitly supplied off-state assumption.
Compare my reasoning
- On-time = 4/4 = 1 h.
- Energy = 2 × 1 = 2 kWh.
- Cost = 2 × 0.30 = 0.60 currency units.
2 kWh; 0.60 currency units.
Look for these in your work
- Use the duty fraction.
- State the off-state model.
The energy charge is 0.30 currency units and a supplied fixed daily charge is 0.10. With no other terms, what is the daily total?
Give me a hint
Add the fixed daily amount once rather than treating it as a per-kWh rate.
Compare my reasoning
- Energy charge = 0.30.
- Fixed daily charge = 0.10.
- Total = 0.40 currency units.
0.40 currency units under the supplied billing terms.
Look for these in your work
- Add the fixed charge once.
- Do not invent extra terms.
Common mistakes
- Price the energy rather than power alone.
- Do not invent real tariffs.
- Separate energy charges from fixed billing terms.
What can you explain now?
What is multiplied by a price per kWh?
Compare with the explanation
Energy measured in kWh.
The kWh unit cancels against the supplied per-kWh price to give a currency amount.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Calculate a supplied energy tariff with operating-time controls and state what billing terms the answer includes.
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.