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Stretch: Southlight Charging: fast chargers on a New Zealand highway
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
1. Read the prompt aloud
Read it slowly, then pause. Let the candidate ask questions before they structure.
First, estimate how many public fast chargers New Zealand needs when 10 percent of its cars and vans are electric. Then: Southlight Charging, a New Zealand charging company, is thinking about building a six-charger site on a main highway. Should it build?
Format note: Difficulty: Stretch. Format: market-sizing opener, then a business question. Industry: Energy and transport. Region: New Zealand. Interview length: about 35 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: Which vehicles, and what share will be electric?
Answer: Cars and vans. Use about 4 million light vehicles (rounded, illustrative) and plan for the point when 10 percent are electric.
If asked: How much energy does an electric car use, and where is it charged?
Answer: About 2,000 kWh a year. About 10 percent of that energy comes from public fast chargers; the rest is charged at home or at work.
If asked: How busy is a fast charger?
Answer: A 150 kW charger is in use, on average, about 10 percent of the hours in a year.
If asked: For the site, what are the costs and prices?
Answer: A six-charger highway site costs NZD 1.2 million, including the grid connection. Southlight sells power at NZD 0.75 per kWh, pays NZD 0.35 per kWh for electricity and network charges, and spends NZD 60,000 a year on other running costs. It wants payback within seven years.
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 cars mostly charge at home, so the need for public fast chargers is modest. My hypothesis is that New Zealand needs a few hundred fast chargers at that point, and that a highway site pays back only if it is busy enough.
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.
- Size the need, then test one site
- Electric vehicles x energy per vehicle x share charged at fast chargers
- Energy one charger delivers in a year
- Key: Site cash flow and payback
- How busy the site must be
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: Electric vehicles
What a strong candidate does: 10 percent of about 4 million light vehicles.
Electric vehicles: 4,000,000 × 0.1 = 400,000
Step 2: Energy from public fast chargers
What a strong candidate does: 400,000 vehicles x 2,000 kWh x 10 percent.
Fast-charging energy (kWh a year): 4,000,000 × 0.1 × 2,000 × 0.1 = 80,000,000
Step 3: Energy per charger
What a strong candidate does: 150 kW, in use 10 percent of the 8,760 hours in a year.
Energy per charger (kWh a year): 150 × 0.1 × 8,760 = 131,400
Step 4: Chargers needed
What a strong candidate does: Total fast-charging energy divided by what one charger delivers: roughly 600.
Fast chargers needed: 4,000,000 × 0.1 × 2,000 × 0.1 ÷ (150 × 0.1 × 8,760) = 609
Step 5: Site cash flow
What a strong candidate does: Six chargers at 10 percent use and a margin of NZD 0.40 per kWh, minus NZD 60,000 of running costs.
Yearly cash (NZD): 6 × 150 × 0.1 × 8,760 × (0.75 - 0.35) - 60,000 = 255,360
Step 6: Payback
What a strong candidate does: NZD 1.2 million divided by the yearly cash.
Payback (years): 1,200,000 ÷ (6 × 150 × 0.1 × 8,760 × (0.75 - 0.35) - 60,000) = 4.7
Step 7: Curveball: a quieter highway
What a strong candidate does: Interviewer: "Traffic data shows this highway is busy on holidays and quiet the rest of the year. Expect only 6 percent use for the first years."
Payback at 6 percent use (years): 1,200,000 ÷ (6 × 150 × 0.06 × 8,760 × (0.75 - 0.35) - 60,000) = 9.29
Step 8: Use needed for a seven-year payback
What a strong candidate does: The site must earn NZD 1.2 million divided by 7 each year, plus its running costs, from a margin of NZD 0.40 per kWh.
Use needed (%): (1,200,000 ÷ 7 + 60,000) ÷ (0.75 - 0.35) ÷ (6 × 150 × 8,760) × 100 = 7.34
The recommendation to listen for
At the end, say: "The CEO walks in. What is your recommendation?"
On these illustrative inputs, New Zealand needs roughly 600 public fast chargers when 10 percent of its cars and vans are electric. Southlight should build this site, but in stages. First, at 10 percent use a six-charger site earns about NZD 255,000 a year and pays back in about 4.7 years. Second, if the highway is quiet outside holidays and use is only 6 percent, payback stretches to about 9.3 years, beyond Southlight's seven-year rule. Third, the site needs about 7.3 percent use to pay back in seven years, so the decision depends on traffic. Install two chargers now with a grid connection sized for six, add chargers as use passes about 7 percent, and look for a host business such as a service station or café to share costs.
Risks a strong answer names: Holiday peaks may leave drivers queuing even when average use is low; Electricity network charges could rise.
Next steps: Get traffic counts by month for the highway; Ask the network company for the cost of a connection sized for six chargers.
Strong versus weak
A strong answer
Built a clear sizing chain, then tested the site on payback and found the use level it needs before recommending a staged build.
A weak answer
Gave a charger count with no steps, then said "build, because electric cars are growing," without testing how busy the site would be.
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?
Total
0 out of 25
Score all five criteria to see the band and the feedback template.