Why charging efficiency matters
When an EV charges, some of the electricity drawn from the outlet is lost as heat during conversion rather than reaching the battery — charging efficiency describes what percentage of the drawn energy actually makes it into the battery. An 85% efficiency means 85% of what you're billed for ends up as usable stored energy, with the remaining 15% lost in the process.
The formula
Energy needed = battery capacity × (target charge % − current charge %) ÷ 100. Actual energy drawn from the grid = energy needed ÷ charging efficiency. Total charging cost = actual energy drawn × electricity rate per kWh. The energy-needed step figures out how much the battery itself gains; the actual-energy-drawn step accounts for the conversion loss on top of that.
A worked example
Charging a 40 kWh battery from 20% to 100% needs 32 kWh to reach the battery (40 × 80 ÷ 100). At 85% charging efficiency and ₹8 per kWh, the actual energy drawn from the grid is 37.65 kWh (32 ÷ 0.85), costing ₹301.18 total. A smaller top-up from 50% to 80% on the same battery needs only 12 kWh, and at a better 90% efficiency and the same ₹8 rate, draws 13.33 kWh for a total cost of ₹106.67.
Why a smaller top-up isn't proportionally cheaper in efficiency terms
Even though the second example charges a smaller percentage of the battery, the higher 90% efficiency assumed for it (versus 85% in the first example) means less energy is wasted per kWh delivered — charging efficiency itself can vary by charger type and charging speed, with slower AC charging generally being somewhat more efficient than fast DC charging.
Why this matters beyond just budgeting
This gap between energy needed and energy actually billed is exactly why a simple "kWh per 100 km × electricity rate" running-cost estimate, like the one in the companion article on this site, can understate the true cost of running an EV if that consumption figure doesn't already account for charging losses — the connection between the two is explored directly in a third companion article.