Skill 04 · Learn fast, and learn from people
How many public EV charging points will a city of one million people need by 2030?
You probably haven’t seen this one before, and that’s the point. The interviewer is testing how you reason when there’s no pattern to copy, and how you take a correction.
The question
A city government is planning its charging network and wants a rough number to budget against. The assumptions below are illustrative, for practice, not forecasts.
“Roughly how many public chargers will the city need by 2030?”
2 min Think first. Jot down your answer, or say it out loud, before you open the steps. The steps make far more sense once you’ve tried it yourself.
A strong answer, step by step
Open each step in turn. This is our worked answer, not the only good one.
Step 1: Define what you’re counting
“I’ll count public charging points, not home chargers, and size for demand on a typical day. I’ll start from the number of electric cars.”
Why it works: A one-line definition avoids sizing the wrong thing, and it tells the interviewer your approach before the math.
Step 2: Build it from cars to chargers
“A million people is about 400,000 households and, say, 450,000 cars. If 20% are electric by 2030, that’s 90,000 EVs. Maybe 40% can’t charge at home, so 36,000 cars rely on public charging.”
Why it works: Each step is one assumption you can defend or change. Round numbers keep the math in your head and the interviewer with you.
Step 3: Turn demand into charging points
“If those cars need a charge about twice a week, at roughly an hour on a fast charger, that’s about 10,000 charging hours a day. A fast charger in use eight hours a day covers eight of those sessions, so about 1,300 fast chargers. Slower kerbside chargers would need several times more.”
Why it works: You link demand to capacity through utilisation, the assumption that matters most. Saying “several times more for slow chargers” shows you know the answer depends on the mix.
Step 4: Sanity-check, and say what you’d check first
“That’s about one fast charger for every 70 EVs in the city. I’d check it against a city that’s further ahead, and the first assumption I’d test is the share of drivers without home charging.”
Why it works: A ratio check catches big errors, and naming the most sensitive assumption is what turns an estimate into advice.
Common traps
- Jumping to a number with no structure.
- Using precise-looking figures you can’t defend.
- Defending an assumption when the interviewer corrects it. Take the new number and move on.