A worked example connecting the two
Take an EV consuming 15 kWh per 100 km, charged from 20% to 100% on a 40 kWh battery at 85% charging efficiency and ₹8/kWh. That charge puts 32 kWh of usable energy into the battery, enough for 213.33 km of driving (32 ÷ 15 × 100). The simple estimate says this driving costs ₹1.20/km (15 kWh/100km × ₹8/kWh ÷ 100). But the charging session itself actually cost ₹301.18 total (accounting for the 85% efficiency loss) — dividing that real cost by the 213.33 km it enables gives a true cost of ₹1.41/km, about 17.6% higher than the simple estimate.
Why the gap is exactly explained by the efficiency percentage
The 17.6% gap isn't a coincidence — it's approximately 1 ÷ 0.85 − 1, the same relationship used directly in the charging-cost formula. Whatever charging efficiency percentage applies to a given charger, dividing the simple per-km estimate by that efficiency (as a decimal) converts it into the true, charging-loss-inclusive cost per km.
When the simple estimate is actually fine to use as-is
If the kWh-per-100km consumption figure being used is already measured "at the wall" (an EV's real-world consumption as tracked by a home charging station or utility meter, rather than the vehicle's own battery-level trip computer reading), charging losses are already baked into that number, and the simple estimate doesn't need a separate efficiency adjustment. The gap described here only applies when the consumption figure reflects energy delivered to the battery rather than energy drawn from the outlet.