Cost per Mile to Charge an EV: How the Number Is Built

Two of the three inputs are specific to your house and your driving, which is why every published figure is somebody else’s arithmetic.

Every quoted figure for what it costs to drive an electric car a mile is
somebody else’s arithmetic done with somebody else’s inputs. The arithmetic
itself is short, and once you have seen it you can do it with your own numbers
in about ten minutes — which is the only version worth trusting, because two of
the three inputs are specific to your house and your driving.

Cost per Mile to Charge: The Three Numbers Behind It

The whole calculation is one multiplication. Take the energy your car pulls
from the wall to travel one mile, and multiply it by what one
kilowatt-hour actually costs you delivered. That is the cost per mile. Every
argument about electric running costs is really an argument about which values
belong in those two slots, and about a third factor sitting quietly between
them.

The first number: energy per mile, as the car uses it

Manufacturers and the federal fuel economy label express consumption as
kilowatt-hours per 100 miles. Divide by 100 and you have kilowatt-hours per
mile. If your car reports the other way round, in miles per kilowatt-hour,
invert it instead. Most passenger electric vehicles land somewhere between
roughly a quarter and a half a kilowatt-hour per mile, with large trucks and
SUVs at the top of that band, but the range is wide enough that the class
average is useless to you. The figure for your specific car is on its window
sticker and on the federal fuel economy site, and your own trip computer’s
lifetime average is better still, because it already contains your driving
rather than a test cycle’s.

That number is not a constant. Highway speed costs more energy per mile than
suburban driving, because drag rises steeply with speed; cold air is denser and
cold tires roll harder; hills, roof racks and a loaded car all move it.

The second number: what a kilowatt-hour actually costs you

This is where most estimates go wrong, and they go wrong in the same
direction every time. People take the supply rate — the per-kilowatt-hour price
on the energy line of the bill or in an ESCO contract — and use that. But a
kilowatt-hour arriving at your charger has been billed twice: once for the
energy and once for delivering it, plus the taxes and surcharges that ride on
both. Using only the supply price produces a cost per mile materially below
what you actually pay.

The number you want is the all-in one, and it is easy to build: take the
total amount charged on a bill and divide it by the kilowatt-hours that bill
covers. That single division absorbs supply, delivery, taxes and riders without
you having to understand any of them individually. Do it across several months
rather than one, because the mix shifts seasonally and because rates in this
state are revised on their own schedule — the reason your own recent bills are
a better source than any published figure, and the subject of
how rate changes work in New York.

One caveat on that division. A fixed monthly customer charge does not change
whether you drive 100 miles or 1,000, so strictly it does not belong in a per
mile figure at all. Including it inflates the cost of every mile and inflates it
most for people who drive least. If you want the marginal cost of a mile — which
is the number that answers “what does this trip cost me” — use the rate that
applies to the next kilowatt-hour, not the average of a bill that contains fixed
charges. If you want the household’s total transport energy cost, the average
is fine.

The third number: the energy that never reaches the battery

The meter counts what leaves the wall. The car reports what arrived in the
battery. Those are not the same quantity, and the gap is real energy you paid
for: it goes into the onboard charger’s conversion losses, into the cable, and
into keeping the battery at a workable temperature while it charges.

The gap is not fixed. It is wider when charging slowly, because the fixed
overhead of running the car’s systems is spread over fewer kilowatt-hours per
hour — one of the less obvious costs of
charging from an ordinary household outlet
rather than a dedicated circuit. It is wider in winter, when energy is spent
warming a cold pack before it will accept a charge. And it is invisible unless
you look for it, which is why a cost per mile built from the car’s own energy
display is always optimistic.

Where each input actually lives

Time-of-use turns one number into two

On a flat rate, every mile costs the same whenever you charge. On a
time-of-use rate it does not, and car charging is the most shiftable large load
a house has: the car sits still all night and does not care when it receives
its energy. That makes it the load that benefits most from a rate that prices
overnight hours differently — the mechanism set out in
who benefits from a time-of-use rate and who pays more,
and the same logic that governs
running battery storage on a time-of-use rate.

If you are on such a rate, your cost per mile is two figures, not one: the
off-peak number for the miles you charged overnight and a higher one for
anything you topped up during the day. Blend them by the share of energy in
each period, not by the share of sessions.

Winter is a separate calculation

New York gives you months where cabin heat runs continuously, the battery
spends energy keeping itself warm, and snow and cold roads raise rolling
resistance. All three push kilowatt-hours per mile up at the same time, and the
charging losses widen alongside them. The seasonal swing in a car’s energy use
is real and is the same shape as the swing in the rest of the house, described
in why a year of bills looks the way it does.
An annual average is the honest figure to quote; a July figure is a best case
presented as a year.

Miles charged away from home are priced differently

Public charging is not sold the way home electricity is sold. Depending on
the network and the location it may be priced per kilowatt-hour, per minute of
connection, or with a session fee on top, and some sites add idle fees once the
car has finished. None of those structures reduce to your household rate, so a
household cost per mile is really a weighted blend: home miles at your all-in
rate, away miles at whatever the session actually charged. Keep the two apart
and blend them by energy, and the figure stays honest.

Comparing it against a gasoline car

The comparison is only fair if both sides are built the same way. For
gasoline, cost per mile is the pump price of a gallon divided by real-world
miles per gallon — not the sticker figure, for the same reason the electric
side should use your own trip computer. Do one side with a supply-only
electricity rate and the other with a sticker economy figure and the comparison
means nothing at all.

Measuring yours instead of estimating it

Estimating gets you close. Measuring settles it, and takes one month of mild
attention:

• Write down the odometer and the charging equipment’s lifetime energy
  counter on the same day.
• Repeat a month later. The difference in the counter is energy at the wall,
  including the losses; the difference in the odometer is miles.
• Divide energy by miles for kilowatt-hours per mile as actually
  delivered.
• Multiply by the all-in price from that month’s bill.
• Subtract nothing for the miles you charged elsewhere — remove those miles
  from the odometer figure instead, or your home rate will absorb them.

A charger on its own circuit makes this far easier, since the equipment
counts only itself and nothing else on the house is mixed in; that separation is
one of the quieter arguments for
giving the charger a dedicated circuit
rather than sharing one.

What solar does and does not change

Panels do not make charging free; they change which price applies. Energy
consumed directly while the array is producing displaces a kilowatt-hour you
would have bought at the all-in rate. Energy drawn at night is bought, and is
then offset by credits your exports earned under the compensation rules that
apply to your system. Whether daytime charging is worth arranging therefore
depends on how your exports are credited, which is the whole substance of
how net metering credits work in New York
and of the practical question covered in
charging a car from a home solar array.

None of this is difficult arithmetic. It is just arithmetic that nobody can
do for you, because two of the three inputs are yours and the third is on a
piece of paper the utility sends you every month.

Frequently Asked Questions

What is the formula for cost per mile to charge?

Kilowatt-hours drawn from the wall per mile, multiplied by your all-in price per kilowatt-hour. The wall figure matters rather than the battery figure, because you pay for the conversion and conditioning losses too, and the all-in price matters rather than the supply rate, because delivery, taxes and surcharges are billed on the same kilowatt-hour.

Should I use my supply rate or my whole bill?

The whole bill. Divide the total amount charged by the kilowatt-hours that bill covers and you get an all-in price that already contains supply, delivery, taxes and riders. Using only the per-kilowatt-hour supply price is the single most common reason a published cost per mile comes out below what people actually pay.

How much energy is lost between the outlet and the battery?

Enough to matter, and the only reliable figure is your own. Compare the kilowatt-hours the charging equipment counted with the kilowatt-hours the car says it received over the same period. The gap is wider when charging at low power and wider in cold weather, because the car’s own systems run for longer per kilowatt-hour delivered.

Does charging at home cost less than using a public charger?

They are priced by different mechanisms, so the only way to know is to compute both. Home energy is billed per kilowatt-hour on your utility rate. Public sessions may be billed per kilowatt-hour, per minute of connection, or with a session fee added, which means the same session can produce very different per-mile figures depending on how fast the car actually accepted the charge.

Does cold weather change the cost per mile?

Yes, in three ways at once. Cabin heating draws energy that does not move the car, the battery spends energy keeping itself in its working temperature range, and cold roads and dense air raise the energy needed per mile. Charging losses widen at the same time, so a winter figure and a summer figure can be noticeably different numbers for the same car.

Should the fixed monthly charge on my bill be included?

Not if you want the cost of an additional mile, because that charge does not change with how far you drive. Including it makes every mile look more expensive and penalises low-mileage drivers most. Include it only if you are working out the household’s total energy cost rather than the marginal cost of driving.

Does having solar panels make charging free?

No, it changes which price applies. Charging while the array is producing displaces energy you would otherwise have bought. Charging at night is bought from the grid and then offset by whatever credits your exports earned, so the value depends on the compensation rules your system falls under rather than on the panels themselves.