Why petrol and EV running costs can't be compared directly
A petrol vehicle's efficiency is measured in kilometers per liter; an EV's efficiency is measured in kilowatt-hours consumed per 100 kilometers. These are different units for different kinds of energy, so comparing them requires converting each vehicle's consumption and local energy price into a monthly rupee cost first, then comparing those two cost figures directly.
The formula for each side
Petrol monthly cost = (monthly distance ÷ km per liter) × price per liter. EV monthly cost = (monthly distance ÷ 100) × kWh consumed per 100 km × electricity price per kWh. The petrol side converts distance into liters consumed before pricing it; the EV side converts distance into "hundreds of kilometers" to match how EV consumption is normally quoted, then prices the kWh used.
A worked example
For 1,500 km of monthly driving, a petrol vehicle at ₹100/liter and 15 km/liter costs ₹10,000/month (1,500 ÷ 15 × 100). An EV at ₹8/kWh and 15 kWh per 100 km costs only ₹1,800/month (1,500 ÷ 100 × 15 × 8). That's a ₹8,200/month saving, or ₹98,400/year — more than double the diesel-vs-petrol savings in the companion article on this site (₹3,250/month, on the identical 1,500 km/month distance), reflecting how much cheaper electricity is per unit of distance than either liquid fuel in this example.
Why the savings gap is usually this large
Electricity is typically priced far below liquid fuel per unit of usable energy, and EV drivetrains convert that energy to motion more efficiently than internal combustion engines — both factors push in the same direction, which is why petrol-to-EV running-cost comparisons tend to show larger savings than a diesel-to-petrol comparison, where both vehicles share the same combustion-engine inefficiency.
What this comparison doesn't include
This is a running-cost-only comparison — it doesn't account for the EV's typically higher upfront purchase price, battery degradation over the vehicle's life, home or public charging infrastructure costs, or the petrol vehicle's own maintenance costs, all of which belong in a full cost-of-ownership comparison alongside the fuel savings calculated here.