Fermi estimate: sizing the electric vehicle fleet on Bangalore's roads

An interviewer asks you for a reasoned estimate of how many electric vehicles Bangalore has. You have no data to look up, so build the number from assumptions you state out loud, show the arithmetic, and finish with a check that the result is plausible. ```hint Split before you multiply The city's electric vehicles are not one population. Who owns them, and why, differs a lot between vehicle types and between private and commercial use, so estimate each group on its own. ``` ### Clarifying Questions - Which vehicles count: only personal cars and two-wheelers, or also electric autos, taxis, delivery fleets and buses? Do low-speed e-scooters that need no registration, and e-bicycles, count? - Is the target the vehicles in active use today, or every electric vehicle ever registered in the city, including ones that have since moved away or been scrapped? - Should the boundary be the city proper or the wider metropolitan area? - What will the number be used for, such as sizing charging demand or a vehicle-finance product? That decides which groups matter most. ### What a Strong Answer Covers - A clear definition of what is being counted before any arithmetic starts. - A segmented structure by vehicle type and by private versus commercial use, written as an explicit equation. - Labeled, round-number assumptions for population, vehicle ownership and electric share, including the difference between the electric share of new sales and the electric share of the vehicles on the road. - Correct arithmetic, a final number and a sensible range. - A sanity check by a second method, and a note on which assumption moves the answer most. ### Follow-up Questions - Which single assumption is your estimate most sensitive to, and how would you get real data for it? - How would you project the number five years out? - If the estimate were for planning public charging stations, which groups would you weight differently, and why?