The formula
Cost per 100 km = energy consumption (kWh per 100 km) × electricity rate per kWh. Cost per km is simply that figure divided by 100. EV consumption is normally quoted in kWh per 100 km, which is why the calculation is built around that unit rather than kWh per km directly.
A worked example
An EV consuming 15 kWh per 100 km, charged at ₹8 per kWh, costs ₹120 to drive 100 km, or ₹1.20 per km. A less efficient EV consuming 18 kWh per 100 km at a lower ₹7 per kWh rate costs ₹126 per 100 km, or ₹1.26 per km — showing that a higher electricity rate isn't the only thing that can make one EV cost more to run than another; higher energy consumption pushes the cost up even when the rate per unit is lower.
Why consumption varies between vehicles and conditions
kWh per 100 km isn't a fixed number for a given vehicle model — it varies with driving style, terrain, temperature (cold weather typically increases consumption), and speed, similar to how a petrol vehicle's fuel economy varies with driving conditions rather than being a single fixed figure.
Why this is a simplified estimate
This calculation treats every kWh of energy consumption as costing the same regardless of how it reached the battery — in reality, charging isn't perfectly efficient, so the actual cost of getting that energy into the battery is somewhat higher than this simple per-consumption estimate suggests. The companion articles on this site cover charging cost and charging efficiency separately, since they answer a related but different question.