Interviewer view · keep this screen to yourself

How a grid battery earns money by shifting energy

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 100 MWh battery in Europe charges when power costs USD 20 per MWh and sells in the evening peak at USD 90 per MWh. It loses 15 percent of the energy in the round trip (85 percent efficiency). It completes 300 full cycles a year. What is its gross margin per cycle and per year, in USD?

2. Answers to clarifying questions

This case has no scripted clarifying answers. Answer from the prompt, and say "assume what you think is reasonable" if the prompt does not cover it.

3. 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.

Margin per cycle = energy sold x peak price minus energy bought x off-peak price
- Energy sold = energy bought x round-trip efficiency
- Yearly margin = margin per cycle x cycles per year

4. 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: Revenue per cycle
What a strong candidate does: 85 MWh sold at 90.
Revenue per cycle (USD): 100 × 0.85 × 90 = 7,650
Step 2: Cost per cycle
What a strong candidate does: 100 MWh bought at 20.
Charging cost per cycle (USD): 100 × 20 = 2,000
Step 3: Margin per cycle
What a strong candidate does: Revenue minus charging cost.
Margin per cycle (USD): 7,650 - 2,000 = 5,650
Step 4: Yearly margin
What a strong candidate does: 300 cycles a year.
Yearly gross margin (USD): 5,650 × 300 = 1,695,000

The recommendation to listen for

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

The owner should go ahead only if the price spread holds, because the battery earns about USD 5,650 per cycle and about USD 1.7 million a year before its running costs and capital charge. First, it sells 85 MWh at USD 90 for USD 7,650 against USD 2,000 to charge. Second, it needs 300 full cycles a year, which means the spread must be large and frequent. The risk is that new batteries narrow the spread. As a next step, check extra income from grid services or capacity payments.

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.