Your monthly EV charging cost comes from three numbers you can find today: kilometres you drive per month, your vehicle's real efficiency in km per kWh, and your electricity tariff in ₹ per kWh. The formula: divide kilometres by efficiency to get the energy your battery needs, divide by charging efficiency (about 0.9, because some energy is lost as heat while charging) to get the energy your meter will actually record, and multiply by the tariff. Our monthly cost calculator does this live; this article shows the working so you can sanity-check any number anyone quotes you.
The formula
| Energy into battery (kWh) | = monthly km ÷ efficiency (km/kWh) |
| Energy from wall (kWh) | = energy into battery ÷ 0.9 |
| Monthly cost (₹) | = energy from wall × tariff (₹/kWh) |
| Cost per km (₹) | = monthly cost ÷ monthly km |
Where each number comes from
- Monthly kilometres: odometer difference across a month beats any guess.
- Efficiency: use your vehicle's displayed average (km/kWh or Wh/km — to convert Wh/km, divide 1000 by it). Real-world efficiency is usually worse than the brochure: traffic, AC, load and driving style all cost energy.
- Tariff: the per-unit energy charge from your electricity bill, at the slab your total consumption lands in — note that adding charging can push you into a higher slab, which raises the marginal rate. Reference tariffs with sources and verification dates are on our state guides.
- Charging efficiency: AC charging typically loses some energy as heat; 90% is a reasonable planning figure and is the labelled assumption our calculators use.
Worked example 1: a mid-size electric car
1,000 km/month, 6 km/kWh, tariff ₹8/kWh:
- Energy into battery = 1000 ÷ 6 = 166.7 kWh
- Energy from wall = 166.7 ÷ 0.9 = 185.2 kWh
- Monthly cost = 185.2 × 8 = ₹1,481 (≈ ₹1.48 per km)
Worked example 2: an electric scooter
600 km/month, 25 km/kWh, tariff ₹7/kWh:
- Energy into battery = 600 ÷ 25 = 24 kWh
- Energy from wall = 24 ÷ 0.9 = 26.7 kWh
- Monthly cost = 26.7 × 7 = ₹187 (≈ ₹0.31 per km)
What this method does not capture
- Slab effects: if charging pushes your total consumption into a higher tariff slab, your marginal cost rises — check your DISCOM's slab structure.
- Fixed charges: a separate charging connection or enhanced load may carry fixed monthly charges the formula ignores.
- Public charging: commercial fast charging is priced per kWh (or per minute) at rates unrelated to your home tariff — budget it separately.
- Seasonal variation: efficiency drops in extreme heat with heavy AC use.
After your first full month of charging, compare the sub-meter or charger reading against the formula's prediction — the difference tells you your real-world charging efficiency and tightens every future estimate. For the installation side of the budget, see the charger cost calculator and who pays for wiring.