EV charging cost:
kWh, charging losses and cost per kilometre

The battery gets less than the meter records

A car battery stores direct current, while the socket supplies alternating current. The onboard charger converts one to the other, and that conversion produces heat. The cable, the battery management system and, in cold weather, battery warming also use some energy. So the figure on your home meter is higher than what ends up stored.

Overall charging efficiency from wall to battery is commonly somewhere in the 85% to 95% range for home AC charging. It differs with the vehicle, charging power and temperature, so use your own figure if you have measured it.

The charging cost formula

If the car reports the energy it received from the charger, that is already the grid-side number and no efficiency division is needed.

When the car reports a charge in percentage terms, convert it first. Charging a 60 kWh pack from 20% to 85% adds 65% of 60, which is 39 kWh to the battery. With 90% efficiency the grid supplies 43.3 kWh, and at ₹9 per unit the session costs about ₹390.

Worked example: adding 40 kWh at home

At a public charger priced at ₹18 per kWh, the same top-up costs 40 × 18 = ₹720 if the operator bills the energy delivered to the car. If the operator bills what its own equipment draws, losses are included and the sum is closer to ₹800. The session receipt shows which basis applies.

A useful sanity test is to compare your result with the car's own energy report. If the dashboard shows 40 kWh added and the wall meter moved by 44 to 45 kWh over the session, your 90% assumption is about right. If the meter moved by 50 kWh, charging is only 80% efficient and something such as cold weather, a long cable run or a weak connection is wasting power.

Turning a charge into a cost per kilometre

Cost per kilometre is the more useful comparison, because it lines up with fuel. Take the car's consumption in kWh per km, divide by efficiency to get the grid-side figure, and multiply by price.

For a car that takes 0.16 kWh per km from its battery and charges at 90% efficiency, the grid energy is 0.16 ÷ 0.90 = 0.178 kWh per km. At ₹9 that is ₹1.60 per km. A petrol car doing 15 km/L at ₹105 per litre costs ₹105 ÷ 15 = ₹7 per km on the same basis.

A monthly budget view

Most drivers think in kilometres a month rather than per charge. At 1,200 km a month and 0.16 kWh per km from the battery, the car draws 192 kWh into the battery, and at 90% efficiency 213.3 kWh from the grid. At ₹9 that is about ₹1,920 a month.

The same distance in a 15 km/L petrol car at ₹105 needs 80 litres and costs ₹8,400. The gap of about ₹6,500 a month is the running-cost advantage, before you weigh the difference in purchase price, insurance and servicing.

Home charging usually adds to your top slab, so the marginal price of the extra 213 kWh can be higher than your average rate. If your slabs rise sharply, use the marginal rate in the formula rather than the bill total divided by units.

Where the price per kWh comes from

• Home charging: your domestic tariff, which may be slab-based. Charging overnight adds to your top slab, so use the marginal rate.
• Public AC and DC chargers: operators set per-kWh or per-minute prices, often with a platform fee and tax.
• Time-of-day tariffs: some utilities offer cheaper night rates for EV charging; check availability with your distribution company.
• Idle or overstay fees: charged when a car stays plugged in after a session ends.

If you have rooftop solar or a time-of-day tariff, your effective price per kWh can be far below the headline rate for part of the day. In that case calculate two scenarios, one at the daytime solar-assisted price and one at the grid price, and use the one that matches when you actually charge.

What changes the real figure

Charging power, battery temperature and state of charge all shift efficiency. Charging from 10% to 80% is faster and usually more efficient per kWh than topping up the last few percent at a slow rate. Cold packs need warming, which takes energy before any charge is stored.

A calculator that takes the battery energy, efficiency and tariff makes it simple to compare home charging with a public session and with the fuel you would otherwise buy.

Keep a simple log for a month, with date, kWh added, meter reading and the amount paid if you use public chargers. The log gives you an average cost per kWh across all your charging and a real figure for efficiency, which is more reliable than any rule of thumb.

Habits that trim the bill

• Charge at home where tariffs allow, and use any off-peak window the utility offers.
• Avoid repeated fast charging when a slow overnight session will do; fast sessions are priced higher and are usually less efficient.
• Precondition the cabin while plugged in so the battery energy is not spent on heating.
• Keep tyre pressure right, since consumption rises with rolling resistance and so does the cost of every charge.

None of these changes the formula. They reduce the energy per kilometre or the price per kWh, which are the two terms that you can influence.