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How to Calculate EV Efficiency and MPGe
Electric-car efficiency is quoted in at least four different units, which makes cars hard to compare at a glance. This guide shows what each unit means, how they convert, and how to turn any of them into MPGe and a real cost per mile.
Answer first: take your car's miles per kWh and multiply by 33.7 to get MPGe; divide your electricity price by miles per kWh to get cost per mile. Everything below is that same arithmetic explained, plus the real-world adjustments that separate a rated number from your actual bill.
The four units you will meet
Manufacturers and reviewers do not agree on one unit, so you will see efficiency written as miles per kWh, kWh per 100 miles, watt-hours per mile, or, outside North America, kWh per 100 km. They all describe the same thing — how far the car goes on a given amount of electricity — but two of them get better as the number rises and two get better as it falls.
Miles per kWh and MPGe are "more is better" figures: a car doing 4 mi/kWh is more efficient than one doing 3. kWh per 100 miles and watt-hours per mile are "less is better" figures, because they measure energy consumed rather than distance covered. Mixing the two mental models is the single most common reason people misread an EV spec sheet.
Converting between the units
The conversions are short division problems. If you know miles per kWh, then kWh per 100 miles is simply 100 divided by that number, and watt-hours per mile is 1000 divided by it. Going the other way, miles per kWh equals 100 divided by the kWh-per-100-miles figure, or 1000 divided by the watt-hours-per-mile figure. To move between miles and kilometres, multiply miles by 1.609 or multiply kilometres by 0.621.
A worked example keeps it concrete. Suppose a sedan is rated at 27 kWh per 100 miles. Divide 100 by 27 and you get 3.70 miles per kWh. Multiply 3.70 by 1000 and you have 270 watt-hours per mile. Multiply 3.70 by 33.7 and you land at roughly 125 MPGe. All four descriptions refer to exactly the same car; only the framing changes.
Where 33.7 comes from
MPGe exists so that an electric car can be compared with a gasoline car on a single window-sticker scale. The US Environmental Protection Agency defines the energy in one US gallon of gasoline as equal to 33.7 kilowatt-hours of electricity. That conversion is an energy equivalence, not a cost equivalence, which is why a car can post an impressive MPGe number and still cost a very different amount to run depending on local electricity and fuel prices. MPGe answers "how efficiently does this vehicle use energy," while cost per mile answers "what will it cost me," and the two questions can point in different directions.
Rated efficiency versus what you actually pay for
The rated figure describes energy measured at the battery, but your home electricity meter sits upstream of the battery. Some energy is always lost as heat in the charger, the cabling, and the battery's own thermal management while charging. These charging losses commonly run between 8% and 15%, and they are higher on slow trickle charging from a standard household outlet than on a proper wall unit. Because you pay for every kilowatt-hour the meter records, your real cost per mile is based on the larger wall figure, not the smaller battery figure. The calculator on the home page models this with a charging-loss field so the cost side reflects reality while the efficiency side keeps the clean rated numbers.
Why winter wrecks your range
Temperature is the biggest swing factor in day-to-day efficiency. In cold weather the battery is less willing to give up and accept charge, cabin heating draws real power because there is no waste engine heat to borrow, and tyre and air resistance both rise. It is normal to see efficiency fall by a fifth or more on a freezing morning, then recover as the pack warms up. Highway speed matters too: aerodynamic drag climbs with the square of speed, so a car that returns 4 mi/kWh around town might only manage 2.8 mi/kWh at a steady 75 mph. None of this means anything is wrong with the car; it is simply physics, and it is why your dashboard average is a better planning number than the sticker.
Turning efficiency into money
Cost per mile is where efficiency becomes a budget. Divide your electricity price in dollars per kWh by your effective miles per kWh, and you have the cost to move the car one mile. At $0.17 per kWh and 3.5 effective miles per kWh, that is about 4.9 cents per mile, or roughly $580 a year over 12,000 miles. Feed the same numbers into a 30-mpg gas car at $3.50 a gallon and you are near 11.7 cents a mile, or about $1,400 a year. The gap between those two figures is the running-cost case for going electric, and it grows or shrinks with your local prices.
How to read your own car's numbers
Every modern EV shows a lifetime or trip efficiency average somewhere in its display, usually as mi/kWh or Wh/mi. That readout, taken over a few weeks of your normal driving, is the most honest input you can give any calculator, because it already bakes in your climate, your route, and your right foot. Note it down, drop it into the tool with your real electricity price, and you will get a cost per mile you can actually trust rather than a laboratory ideal.
Ready to put it to work? Use the EV efficiency calculator with your own dashboard figure, then read electric vs gas cost per mile to see exactly where the savings land.