How to calculate your monthly EV charging cost (with worked examples)

Your monthly EV charging cost comes from three numbers you can collect this week, plus one assumption. Take the kilometres you drive in a month, divide by your vehicle's real efficiency in kilometres per kWh, divide again by charging efficiency (roughly 0.9, because some energy becomes heat instead of range), and multiply by your electricity tariff in rupees per unit. That gives the rupees your meter will actually bill you for. Our monthly charging cost calculator runs the same arithmetic live, but doing it once by hand is worth the ten minutes — after that you can tell instantly when a quoted figure cannot be right.

The formula, one line at a time

Four lines, in order, each feeding the next. The only step people usually skip is the second one, and it is the step that separates a number matching your bill from one that does not.

Why divide by 0.9 rather than multiply? Because you are working backwards. The battery needs a certain amount of energy, so the wall must supply more than that — the charger, cable and battery each give up a little as heat. Dividing by a figure below 1 makes the wall number larger, which is the direction reality moves in.

Where each number really comes from

The formula is trivial. Honest inputs are the actual work, and this is where most estimates go wrong.

• Monthly kilometres. Read your odometer on the same date each month instead of estimating from your commute. Almost everyone underestimates, because the weekend trip to a relative's house and the airport run never make it into the mental average.
• Efficiency in km/kWh. Use your vehicle's own display, not the brochure figure, and take the long-term average rather than one flattering trip. If the display shows Wh/km, divide 1,000 by it — 160 Wh/km is 6.25 km/kWh. Real efficiency usually sits below the advertised number, because traffic, air-conditioning, load and driving style all cost energy.
• Tariff in ₹ per unit. Take the per-unit energy charge from your own bill, not a figure quoted in a WhatsApp group. Most residential bills are slabbed, so use the rate applying to the last units you consume — that is what extra charging gets billed at. Add any per-unit duty or surcharge, and set the fixed monthly charge aside for now. Our guide to reading an electricity bill for EV charging walks through the line items.
• Charging efficiency. Around 0.9 is a reasonable planning assumption for ordinary AC home charging; very slow charging through a portable unit tends to sit slightly lower. Treat it as a starting figure you will replace with your own measurement after a month.

Worked example: a mid-size electric car

These figures are illustrative — substitute your own. Say you drive 1,000 km a month, your display shows a long-term average of 6 km/kWh, and your bill shows a marginal energy charge of ₹8 per unit (check your own bill; rates and slabs differ by DISCOM and state).

• Energy into the battery = 1,000 ÷ 6 = 166.7 kWh
• Energy from the wall = 166.7 ÷ 0.9 = 185.2 kWh
• Monthly cost = 185.2 × 8 = ₹1,481
• Cost per kilometre = 1,481 ÷ 1,000 = about ₹1.48

Notice two things. Adding 185 units is enough, in many slab structures, to push the month's last units into a higher band. And the annual figure — roughly ₹17,800 at these illustrative numbers — is the one to weigh against installation spend using the charger cost calculator.

Worked example: an electric two-wheeler

Same method, much smaller numbers. Say 600 km a month, 25 km/kWh, and an illustrative ₹7 per unit.

• Energy into the battery = 600 ÷ 25 = 24 kWh
• Energy from the wall = 24 ÷ 0.9 = 26.7 kWh
• Monthly cost = 26.7 × 7 = ₹187
• Cost per kilometre = 187 ÷ 600 = about ₹0.31

Around 27 units a month can vanish inside normal household variation, which is precisely why societies argue about whether metering two-wheelers is worth the trouble — an argument better had with numbers in hand.

Check the estimate against a real meter reading

An estimate you never verify stays an estimate forever. After your first full month of charging, close the loop:

1. On day one, write down two numbers: the sub-meter or charger's cumulative kWh reading, and your odometer.
2. On the same date next month, write both down again.
3. Subtract. You now have actual units drawn from the wall and actual kilometres driven.
4. Divide kilometres by units. That is your true km per kWh measured at the wall, and it already contains your charging losses, your driving and your climate.
5. From then on: kilometres ÷ that number × your tariff. No 0.9 assumption needed.

If metered units come out far above what the formula predicted — say 30 per cent or more — do not write it off as "losses". Check whether anything else shares that circuit, whether the charger draws standby power between sessions, and whether the reading period matches the odometer period. A large unexplained gap deserves an electrician's eyes, not a spreadsheet adjustment.

Estimation mistakes that quietly distort the number

• Using the brochure efficiency. Advertised figures come from standardised test conditions. Your basement ramp, city traffic and summer air-conditioning are not those conditions.
• Using the wrong tariff. Total bill divided by total units gives an average rate that includes fixed charges — useful for understanding the past, misleading for predicting the extra cost of charging. Use the marginal slab rate.
• Forgetting the slab jump. A few hundred extra units can move a household into a higher band; our guide on the impact of EV charging on your electricity bill covers this properly.
• Ignoring fixed charges. A dedicated connection or an enhanced sanctioned load may carry recurring fixed charges regardless of usage, and the per-unit formula cannot see them.
• Assuming a bigger charger costs more to run. Charger power sets how fast energy flows, not how much you need — see how home charging time actually works.
• Mixing home and public charging. Commercial charging is priced on its own terms. Estimate the two separately, then add.
• Assuming an off-peak rate you do not have. Some connections are billed differently at different hours and some are not — confirm before planning around it. See night charging and off-peak rates.

What the formula deliberately leaves out

This calculation answers one question: what the electricity for your driving costs each month. It does not price installation, cabling, a sub-meter, or shared infrastructure. Nor does it capture an administrative margin — if your point sits behind a common meter and the committee bills at a rate including a maintenance component, your effective per-unit cost is higher than your flat's tariff. Since the meter your point sits behind changes everything, settle the own-meter, sub-meter or common-meter question first, and committees pricing for several residents should start with the complete guide to billing EV charging fairly.

Finally, treat all of this as arithmetic rather than an electrical opinion. Where your estimate depends on sanctioned load, a new circuit or a meter position, have a licensed electrician assess the site, and confirm slab structures, any time-of-day rates and the current load-enhancement procedure and fees directly with your DISCOM — these differ by state and utility and change over time.

Common questions

How many units of electricity does an electric car use per month?

It depends entirely on distance and efficiency, so there is no universal figure. Using the illustrative numbers above, a car driven 1,000 km a month at 6 km/kWh draws roughly 185 units from the wall, and half that distance draws roughly half. The reliable way to know your own answer is a month of sub-meter or charger readings rather than any published average.

Does a 7.2 kW charger cost more per month to run than a 3.3 kW one?

No — not in any way that shows on your bill. Charger power decides how quickly energy flows into the battery, not how much energy your driving requires, so the same kilometres need roughly the same units either way. Faster charging may lose marginally less to overhead, but that difference is small beside distance and driving style.

Should I use my flat's tariff or my society's common-area rate?

Use the rate for whichever meter your charging point is actually wired behind. If it sits on your flat's connection, use your flat's slab rate. If it sits on the society's common connection, your cost is whatever rate the committee bills at, which may differ from your domestic rate and may fold in a share of fixed costs. Confirm the wiring before assuming either.

Why is my actual bill higher than my calculation?

Usually slab movement, fixed charges, per-unit duties or surcharges the formula ignored, or simply that non-EV household consumption changed the same month. Compare the bill's line items against your assumptions first. If the metered charging units themselves do not match your charger's own readings, that is a wiring or metering question for a licensed electrician, and anything about slabs, surcharges or connection type belongs with your DISCOM.