A kilowatt (kW) measures power, the rate at which energy moves. A kilowatt-hour (kWh) measures energy, the amount moved or stored. A battery's size is a kWh number; a charger's speed is a kW number; and the time to fill the battery is roughly kWh divided by kW.
Key takeaways
- Battery capacity is in kWh, and the figure that matters for driving is the usable capacity, which can be lower than the gross figure.
- AC charging speed is capped by the lower of the charger's kW and the car's onboard charger kW; DC fast charging power depends on the station and on the car's own limits and battery state of charge.
- EPA lists efficiency as kWh per 100 miles (consumption) and as MPGe, where 33.7 kWh equals one gallon of gasoline; kWh/100 miles ties directly to cost.
- The latest EIA national average residential price (August 2026) was 18.16 cents per kWh, so 30 kWh/100 miles works out to about 5.4 cents per mile before any local rate differences.
Power versus energy in plain terms
Think of kW as the speed and kWh as the distance. A 7 kW charger running for one hour delivers 7 kWh. A motor drawing 70 kW for half an hour uses 35 kWh. The relationship is one line:
energy (kWh) = power (kW) × time (hours)
time (hours) = energy (kWh) ÷ power (kW)
- energy (kWh) is the amount of electricity stored in the battery or moved during charging or driving.
- power (kW) is the rate at which that energy flows at a given moment.
- time (hours) is how long the flow lasts.
The formula gives a floor, not a forecast. It assumes the power stays constant and ignores charging losses, and neither assumption holds in real charging, as the sections below show.
Usable vs gross battery capacity
Battery capacity in kWh is the car's fuel tank. Spec sheets can quote it in more than one way. Some manufacturers publish a usable figure, meaning the energy the driver can actually draw, and state it as such: Ford's announcement for the 2027 Mustang Mach-E, for example, says the Select and Premium trims offer 88 kWh of usable capacity. A larger gross or nominal number, where a maker quotes one, can include energy the car holds back, so the two figures are not interchangeable.
When comparing cars, compare like with like: usable with usable. If a listing gives one unlabeled kWh figure, check the manufacturer's own specification page for which one it is.
Two charging limits: AC and DC
The Department of Energy's Alternative Fuels Data Center (AFDC) lists three charging categories with their typical power and the range they add, as of the page checked on October 28, 2026:
| Type | Power | Range added (AFDC estimate) |
|---|---|---|
| Level 1 (AC, 120 V) | 1.9 kW assumed | About 5 miles per hour |
| Level 2 (AC, 240 V home or 208 V commercial) | 2.9 to 19.2 kW; most home units up to 7.2 kW (30 A) | About 25 miles per hour |
| DC fast charging | Up to 500 kW per unit; varies by vehicle and battery state of charge | About 100 to 200+ miles per 30 minutes |
Source for the table: AFDC, electric vehicle charging stations.
The AC ceiling is the smaller of two numbers
On AC charging, the wall unit supplies power and the car's onboard charger converts it for the battery. AFDC lists "the vehicle's internal charger capacity" among the factors that determine charging time, and EPA notes that its MPGe values include losses from the charging cable and the onboard charger. The practical result is that the slower of the two sets the pace.
Illustrative example (round numbers, not a specific model): a car with an 11 kW onboard charger plugged into a 7.2 kW home unit charges at no more than 7.2 kW. The same car on a 19.2 kW unit still tops out at 11 kW. For this car, paying for a wall unit bigger than 11 kW adds no speed.
DC fast charging has its own limits
A station's kW rating is a maximum for the unit, not a promise for every car. AFDC says charging power on DC fast chargers "varies by vehicle and battery state of charge." A car that accepts 150 kW at its peak cannot draw more than 150 kW from a 350 kW station. Where a maker publishes both, the car's DC peak kW and its onboard AC charger kW are separate numbers; do not mix them up.
Why charging slows near full
Fast-charging claims usually name a window rather than a full charge. The fueleconomy.gov overview of EV technology says a fast charge to 80% capacity can take about 30 minutes, while fully recharging the pack can take 3 to 12 hours. AFDC ties DC charging power to the battery's state of charge. Together, these mean the kW delivered is not constant across a session, so the earlier time formula needs an average power, not the peak.
Illustrative example (hypothetical 75 kWh usable battery, 150 kW peak): going from 10% to 80% adds 70% of 75 kWh, or 52.5 kWh. At a constant 150 kW that would take 21 minutes. If the session takes 35 minutes, the average power was 52.5 kWh ÷ 35 min × 60 = 90 kW, which is 60% of the peak. The peak number tells you the best moment of the session; the average tells you the session time. When a maker publishes a charging window (such as 10-80%) and the time to complete it, those two figures give you that average.
Efficiency: kWh/100 miles and MPGe
EPA's label gives efficiency two ways. The fueleconomy.gov label guide says the consumption value tells you how many kilowatt-hours the vehicle would use to travel 100 miles, and that a consumption metric allows more accurate energy-use comparisons among vehicles than miles per gallon does. MPGe converts the same data to a gasoline-style figure.
EPA's fuel economy testing page defines the conversion: a gallon of gasoline has the energy equivalent of 33.7 kilowatt-hours of electricity, so a vehicle using 33.7 kWh to drive 100 miles has an MPGe of 100. The same page says MPGe values include charging losses, assume Level 2 AC charging, and are measured at the wall outlet to reflect what owners pay.
MPGe = 33.7 ÷ (kWh per 100 miles ÷ 100)
= 3,370 ÷ (kWh per 100 miles)
- MPGe is miles per gallon of gasoline-equivalent energy.
- kWh per 100 miles is the label's consumption figure, measured at the wall.
- 33.7 is EPA's kWh-per-gallon conversion.
Illustrative example: 30 kWh/100 miles gives 3,370 ÷ 30 = about 112 MPGe. Lower kWh/100 miles is better; higher MPGe is better. Because the label figure is measured at the wall, it is the number to multiply by your electricity price.
Home charging cost math
The U.S. Energy Information Administration (EIA) reports the national average residential price in its Electric Power Monthly. In its average price table for residential customers, the August 2026 figure is 18.16 cents per kWh, up from 17.57 cents in August 2025 (about 3.4% higher). State and utility rates differ, and time-of-use plans can price overnight kWh differently, so replace the national figure with the rate on your bill.
yearly cost = (miles per year ÷ 100) × (kWh per 100 miles) × (price per kWh)
- miles per year is your annual driving.
- kWh per 100 miles is the wall-measured consumption figure.
- price per kWh is your all-in rate in dollars.
| Step (illustrative inputs) | Result |
|---|---|
| Energy per year: 12,000 miles ÷ 100 × 30 kWh | 3,600 kWh |
| Cost per year: 3,600 kWh × $0.1816 | $653.76 |
| Cost per mile: 30 kWh ÷ 100 × $0.1816 | about 5.4 cents |
| One full fill of a 75 kWh battery at the wall, assuming 90% charging efficiency: 75 ÷ 0.90 × $0.1816 | about $15.13 (about $13.62 if no losses) |
The 90% figure sits inside the 84% to 93% range that fueleconomy.gov says is common for battery charging efficiency. Public fast charging is priced by whoever runs the station, at its own rates rather than your home rate, so this home-rate math does not carry over to it. Educational illustration only; your rates and driving will differ.
Frequently asked questions
Does a 350 kW charger always deliver 350 kW?
No. AFDC says a DC charging unit can provide up to 500 kW, but charging power varies by vehicle and by the battery's state of charge. A car sets what it accepts at a given moment, so a station's rating is a ceiling, not a guaranteed speed.
Can I charge from a regular 120 V outlet?
Yes. AFDC lists Level 1 at about 5 miles of range per hour, assuming 1.9 kW, which is roughly 40 miles after 8 hours for a mid-size EV. That covers short daily distances. AFDC's Level 2 estimate of about 25 miles per hour is five times faster, which matters if daily driving is longer than an overnight Level 1 session can replace.
Do EVs lose energy while charging?
Yes. fueleconomy.gov says battery charging efficiency varies but is often 84% to 93%. For a 75 kWh refill, that means roughly 80.6 to 89.3 kWh drawn from the wall (75 ÷ 0.93 and 75 ÷ 0.84).
Does cold weather change the numbers?
It can. EPA's electric vehicle myths page cites AAA research from 2019 that found range could fall by about 40% on average from cold temperatures and heater use. EPA's testing page also lists a cold test at 20°F, with heat and defrost on, among its five general test cycles. Battery kWh does not shrink on a spec sheet; what changes is how many miles each kWh delivers.
Is EPA range calculated from battery kWh?
No, it is measured by driving. EPA's testing page describes running a fully charged vehicle until the battery is depleted, then, for the most common approach, applying a 0.7 adjustment factor, so 200 highway test miles becomes 140 label miles. City and highway ranges are then weighted 55% and 45% for the combined figure.
Checklist before you compare EVs
- Write down usable kWh for each car, not an unlabeled battery figure.
- Note the onboard charger kW and the DC peak kW as separate numbers, plus the 10-80% charging time.
- Check your home circuit and charger kW against the onboard charger; the smaller number sets AC speed.
- Compare kWh per 100 miles, then multiply by the rate on your latest utility bill.
- Check the manufacturer's page and the EPA label on fueleconomy.gov for the exact model year and trim; figures change between model years.
Sources
- U.S. Department of Energy, Alternative Fuels Data Center: electric vehicle charging stations
- U.S. EPA: Fuel Economy and EV Range Testing
- fueleconomy.gov: Learn more about the fuel economy label
- fueleconomy.gov: All-electric vehicles, how they work
- U.S. Energy Information Administration: Electric Power Monthly, average price of electricity to residential customers
- U.S. EPA: Electric Vehicle Myths
- Ford Motor Company: 2027 Mustang Mach-E Trail announcement
