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Stretch: Electrifying a mine's haul trucks

You run the case. Read the prompt, answer questions from the notes below, and share data only when the candidate asks for it or gets stuck. Score at the end.

Case timer

00:00

Target length:

1. Read the prompt aloud

Read it slowly, then pause. Let the candidate ask questions before they structure.

A global mining company is considering replacing its 50 diesel haul trucks at one mine with battery-electric trucks. Should it?

Format note: Difficulty: Stretch. Format: candidate-led, with interviewer dialogue. Industry: Mining and energy. Region: Global (a mine in Australia or South America). Interview length: about 40 minutes. The company is fictional and all figures are illustrative.

2. Answers to clarifying questions

Give these answers only if the candidate asks. If they ask something not listed, give a sensible answer or say it does not matter here.

If asked: How many trucks, and how long will the mine run?
Answer: 50 haul trucks; the mine has about 15 years of life left.
If asked: What does an electric truck cost and save?
Answer: USD 3 million more per truck, plus USD 40 million of charging equipment for the site. It saves about USD 500,000 a year in energy and USD 100,000 in maintenance per truck.
If asked: What carbon price applies?
Answer: The company's internal carbon price of USD 50 per tonne; each diesel truck emits about 2,000 tonnes a year.

3. The hypothesis a strong candidate states

Listen for an early, testable guess like this one. It does not need to match word for word.

Electric haul trucks save fuel and carbon costs but need large upfront spending. My hypothesis is that payback is around five to six years, and that the source of electricity matters as much as the trucks.

4. A model structure

Compare the candidate's structure with this one. A different split can be just as good if it is clean and fits the problem.

Upfront cost per truck versus yearly savings
- Upfront: extra truck cost plus a share of charging
- Key: Savings: energy, maintenance, carbon
- Payback versus mine life
- Where the electricity comes from

5. The working, step by step

Each step shows how a strong candidate works it out. Share a new fact from it only when the candidate asks or is stuck, and let them do the math: the result in the dark box is what they should reach.

Step 1: Yearly saving per truck
What a strong candidate does: Candidate: "Energy, plus carbon avoided at USD 50 per tonne, plus maintenance."
Saving (USD a year): 500,000 + 2,000 × 50 + 100,000 = 700,000
Step 2: Upfront cost per truck
What a strong candidate does: Extra truck cost plus one fiftieth of the charging equipment.
Upfront per truck (USD): 3,000,000 + 40,000,000 ÷ 50 = 3,800,000
Step 3: Payback
What a strong candidate does: Well inside the 15 years of mine life.
Payback (years): (3,000,000 + 40,000,000 ÷ 50) ÷ (500,000 + 2,000 × 50 + 100,000) = 5.43
Step 4: Fleet upfront cost
What a strong candidate does: 50 trucks plus the charging equipment.
Fleet upfront (USD): 50 × 3,000,000 + 40,000,000 = 190,000,000
Step 5: Curveball: diesel-generated power
What a strong candidate does: Interviewer: "The mine is remote. Its electricity comes from diesel generators, so diesel use and emissions fall only about 30 percent, and the energy saving is only about USD 225,000 a truck." Candidate: "Then both savings shrink, and payback becomes:"
Payback with diesel power (years): 3,800,000 ÷ (225,000 + 2,000 × 0.3 × 50 + 100,000) = 10.7
Step 6: Adding a solar and battery plant
What a strong candidate does: Interviewer: "A partner would build solar with storage and sell us power, raising the energy saving to USD 600,000 a truck and cutting emissions 80 percent." Candidate: "Payback becomes:"
Payback with solar power (years): 3,800,000 ÷ (600,000 + 2,000 × 0.8 × 50 + 100,000) = 4.87

The recommendation to listen for

At the end, say: "The CEO walks in. What is your recommendation?"

Electrify the trucks, together with a solar and storage power supply. First, on clean power each truck pays back in about 5.4 years, well within the mine's 15-year life. Second, on today's diesel-generated power most of the energy and carbon savings disappear and payback stretches to about 10.7 years, too close to the mine's remaining life, so a clean power supply is essential. Third, buying solar power through a partner raises energy savings and cuts emissions 80 percent, bringing payback under 5 years. Replace trucks as the diesel fleet reaches the end of its life rather than all at once, to spread the USD 190 million cost and learn from the first trucks.

Risks a strong answer names: Battery trucks are newer technology in mining; reliability must be proven; Carbon prices may change; Solar output varies, so storage and backup are needed.

Next steps: Run a trial with five electric trucks; Tender a long-term solar and storage power contract.

Strong versus weak

A strong answer

Built the per-truck business case, compared payback with mine life, caught that diesel-generated power undermines the carbon benefit, and fixed the power source.

A weak answer

Assumed electric trucks remove all emissions without asking where the electricity comes from.

Score the candidate

Score each criterion from 1 to 5. A 2 or a 4 sits between the descriptions.

This case has no exhibit. Score Exhibit reading on how the candidate used the data you gave them: did they pick out the number that matters and say what it means?

Structuring

1: No clear structure, or a memorized framework that does not fit the problem.
3: A sensible structure, but with some overlap, a gap, or no clear priority.
5: A structure built for this problem, with no overlap and no gaps, a stated hypothesis, and a clear first step.

Math

1: Frequent errors, no units, or needed heavy help to finish.
3: Right answers but slow or messy working, or one error caught late.
5: Accurate and clearly narrated, with units, a sense check, and what the number means.

Exhibit reading

1: Read the chart aloud without finding the number that matters.
3: Found the key number but gave only part of the so-what.
5: Pulled the right number quickly and said what it means for the case.

Synthesis

1: Summarized what was done, with no clear answer.
3: Gave an answer, but with weak reasons or no next step.
5: Answer first, then two or three reasons with numbers, the main risk, and next steps.

Communication

1: Hard to follow: long silences, no signposting.
3: Clear most of the time, with some rambling.
5: Calm and concise, signposted each step, and checked in with the interviewer.

Total

0 out of 25

Score all five criteria to see the band and the feedback template.