The shape of the lesson
By the end of the lesson, learners can
- Add up the units: Set an appliance power and running time and watch the kilowatt-hours and the cost add up.
- Units, then cost: Work out the energy in kilowatt-hours first, then multiply by the price.
- Energy: kWh = power in kW × time in hours.
- Cost: cost = number of kWh × price per unit.
- Convert first: watts to kilowatts (÷ 1000), minutes to hours (÷ 60).
One picture every idea returns to
The kilowatt-hour is the energy used by a one kilowatt appliance in one hour. Energy in kilowatt-hours is power in kilowatts multiplied by time in hours, and the cost is that energy multiplied by the price of one kilowatt-hour.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, What the bill counts.. The energy company does not bill you in joules, the numbers would be enormous. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Add up the units | Set an appliance power and running time and watch the kilowatt-hours and the cost add up. Project tab 8 of the lab (Power and energy) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Units, then cost | Work out the energy in kilowatt-hours first, then multiply by the price. Model the worked example Worked example on the board, one line at a time. | Rally Coach |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 14 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Units, then cost, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Rally Coach
Pairs share one worksheet. One partner solves the first question aloud while the other coaches, checking each line and offering a prompt rather than the answer; they swap roles for the next question.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| The kilowatt-hour is a unit of energy, not power. | Use the power in kilowatts and the time in hours, then multiply the kilowatt-hours by the price per unit to get the cost. |
Kilowatt-hours and the cost of electricity: worksheet
6 questions, 14 marks. Written in the style of the papers; no past paper question is reproduced.
State what is meant by a kilowatt-hour, and state whether it is a unit of power or of energy.
A 3 kW electric shower is used for 0.5 hours. Calculate the energy it transfers in kilowatt-hours.
Electricity costs 30 cents per kilowatt-hour. Calculate the cost of using 6 kWh of energy.
A 100 W lamp is left on for 4 hours. Calculate the energy used in kilowatt-hours, and the cost at 30 cents per kWh.
Show that one kilowatt-hour is equal to 3.6 × 10⁶ J.
A family runs a 2 kW heater for 2 hours and a 0.5 kW television for 4 hours in one evening. At 30 cents per kWh, calculate the total cost.
Answer key and marking notes
- A kilowatt-hour is the energy transferred by a 1 kW appliance running for 1 hour. ✓
- It is a unit of energy. ✓
- E = P × t = 3 × 0.5
E = 1.5 kWh
- cost = kWh × price = 6 × 30
cost = 180 cents = $1.80
- P = 100 W = 0.1 kW
E = 0.1 × 4 = 0.4 kWh
cost = 0.4 × 30 = 12 cents
E = 0.4 kWh, cost = 12 cents
change watts to kilowatts first
- E = Pt = 1000 W × 3600 s
E = 3 600 000 J = 3.6 × 10⁶ J
1 kW = 1000 W, 1 h = 3600 s
- heater: 2 × 2 = 4 kWh
television: 0.5 × 4 = 2 kWh
total = 6 kWh, cost = 6 × 30 = 180 cents
total cost = $1.80
1 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
A 2.0 kW heater runs for 3.0 hours. Electricity costs 12 pence per kilowatt-hour. What is the cost?
- A72 pence
- B6.0 pence
- C24 pence
- D8.6 pence
Quiz key
Energy = 2.0 × 3.0 = 6.0 kWh, so cost = 6.0 × 12 = 72 pence. Option C forgets to multiply by the time in the energy step.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.