One letter separates kW from kWh, and that letter carries the entire meaning. kW answers “how fast is the water flowing?” kWh answers “how much water is in the tank?”
Read a spec sheet with those two swapped and every number on it will point you the wrong way.
The water analogy that actually works
Imagine filling a tank with water from a hose:
- kW (kilowatt) is the flow rate: how fast water leaves the hose. Bigger hose, higher kW.
- kWh (kilowatt-hour) is the total amount delivered. It is what sits in the tank once you shut the hose off.
Multiply flow rate by time and you get total amount. Electricity behaves the same way:
Energy (kWh) = Power (kW) × Time (hours)
A 7 kW charger running for 8 hours puts 56 kWh into the battery. A 150 kW DC fast charger running for 20 minutes puts 50 kWh into the battery (150 × 0.333 = 50). Wildly different rates, comparable deliveries.
Where you see each in practice
kW shows up on:
- Charging station signs (“150 kW DC Fast Charger”)
- Car spec sheets (“11 kW onboard charger”)
- Power-flow displays during a session (“currently drawing 73 kW”)
- The first number anyone quotes when talking about charging speed
kWh shows up on:
- Battery capacity (“75 kWh battery”)
- Energy bills (“you used 350 kWh this month”)
- Charging session receipts (“delivered: 28.4 kWh”)
- Efficiency claims (“3.5 mi/kWh”)
Once you know which unit a line item is in, the rest of a data sheet stops being intimidating. Reading EV charger specifications (coming soon) walks through the other numbers on the same page.
Why this matters for time-to-charge math
The most useful thing you can do with this distinction: estimate how long charging will take.
Formula: Time (hours) = Energy needed (kWh) ÷ Charging rate (kW)
Examples:
- 60 kWh battery, 20% → 80% (so 60 × 0.6 = 36 kWh needed), on a 7.2 kW Level 2 charger: 36 ÷ 7.2 = 5 hours
- Same charge, on a 150 kW DC fast charger: 36 ÷ 150 = 0.24 hours = 14 minutes (before taper)
- Same charge, on a 1.4 kW Level 1 wall outlet: 36 ÷ 1.4 = 26 hours
This is also why the gap between “11 kW” and “7.2 kW” Level 2 chargers matters less than it sounds. The math is linear: 11 kW finishes in 3.3 hours, 7.2 kW finishes in 5 hours. Both are overnight charges. That single formula is doing most of the work behind how long it takes to charge an EV at any power level.
The charging time calculator handles this for you, including the realistic taper above 80% SOC on DC fast chargers.
The “real-world” complication: taper
The formula above gives the ideal time. The real number runs longer, for three reasons.
1. The battery doesn’t accept full power at high SOC
Above roughly 80% state of charge, the battery management system starts cutting the current it asks for. Cells sit closer to their voltage ceiling as they fill, and forcing high current into a nearly full cell risks plating lithium metal on the anode, so the BMS trades speed for cell life.
The visible result on a DC fast charger: the second half of the session is far slower than the first. Getting from 80% to 100% can take longer than 20% to 80% did.
2. The car has its own kW cap
A car with a 7.2 kW onboard AC charger plugged into an 11 kW station will only draw 7.2 kW. The station has the headroom; the car cannot use it.
For DC fast charging the same logic applies on the DC side: a car capped at 100 kW DC gets 100 kW from a 350 kW station, not 350. The reason the cap lives in a different place for AC and DC is worth knowing, and AC vs DC charging (coming soon) covers where the conversion hardware actually sits.
3. Temperature changes everything
Cold cells have higher internal resistance and slower lithium diffusion, so the BMS limits current until the pack warms up. Preconditioning (coming soon) is the counter: when the car’s route planner knows a DC stop is coming, it heats the pack on the way so it arrives ready to take full power. Setting the charger as a navigation destination, rather than just driving to it, is what gives the car a reason to start heating.
The signs that catch newcomers
A few specific places people get confused:
“150 kW” on a charger sign isn’t a promise. It’s the maximum the station can deliver. The car decides what it actually pulls. Reading “150 kW” and expecting any car to get 150 kW is wrong.
A quoted kWh figure isn’t automatically usable capacity. Packs carry two numbers: gross, meaning what the cells physically hold, and usable, meaning what the BMS lets you access after reserving a buffer at each end of the range. That buffer exists to protect cell life and to keep the range estimate from ever hitting a true zero. Which figure appears on a spec sheet is not standardized, so before you compare two cars, make sure you have the same one on both sides.
Electricity rates on your home bill are in kWh. When you weigh home charging against public charging, put $/kWh on both sides. Comparing $/kWh of home electricity to $/minute of DC fast charging compares two different units, and the conversion between them depends on the kW your specific car is pulling.
How efficient is your EV?
EV efficiency is reported in miles per kWh (or kWh per 100 miles, in some regions). It lands in a few recognizable bands, and what moves a car between them is mass, frontal area, tire rolling resistance, and how much energy the cabin heater is drawing.
| Efficiency | Typical of | Means |
|---|---|---|
| 5+ mi/kWh | Light, aerodynamic cars driven gently in mild weather | A small battery goes a long way |
| 3.5-4.5 mi/kWh | Mainstream sedans and compact crossovers | The usual band |
| 2.5-3.5 mi/kWh | Large SUVs and pickups, performance trims, big wheels | Energy-hungry |
| <2.5 mi/kWh | Towing, sustained highway speed, deep cold | Worst case |
Multiply efficiency by battery size to get range. A 75 kWh pack at 3.5 mi/kWh is 262 miles. The same pack at 2.7 mi/kWh, which is what a winter highway run does to a car, is 202 miles. Same battery, two different cars as far as the range display is concerned. Efficiency in kWh per mile (coming soon) goes deeper on what drives that number.
The cost reframe
“Is EV charging cheaper than gas?” cannot be answered until both sides are in the same units:
- Gasoline car: gallons per 100 mi × price per gallon = $/100 mi
- EV: kWh per 100 mi × price per kWh = $/100 mi
Run it with numbers you can verify off your own bill and your own local pump price. Using $3.50/gallon and a 28 MPG car: (100/28) × 3.50 = $12.50/100 mi. Using $0.15/kWh and 3.5 mi/kWh: (100/3.5) × 0.15 = $4.30/100 mi. Swap in a public fast-charging rate of $0.40/kWh and the EV side climbs to $11.40/100 mi, close enough to the gas figure that the advantage nearly vanishes.
The inputs are yours to supply. What the formula hands you is the structure: an EV’s cost per mile is set by exactly two things, its efficiency and the price of the electricity, and moving from home charging to public fast charging changes only the second one. The cost calculator does the arithmetic.
The unit you are billed in decides who wins
Here is the part that never makes it onto a spec sheet. Public charging can be priced per kWh or priced per minute, and those are not two ways of selling the same thing. Per-kWh pricing sells you energy. Per-minute pricing sells you time on the connector, which means you are paying for kW whether or not your car can take them.
Put two cars on the same per-minute station, one pulling 150 kW and one capped at 50 kW. They pay the same per minute and leave with very different amounts of energy, so the slower car pays roughly three times as much per kWh for an identical product. Taper sharpens the effect: the tail end of a per-minute session is the most expensive energy you will ever buy, because the clock keeps running while the current falls away.
The same split reappears on the commercial side of the meter. A household bill is dominated by kWh, but a site with DC chargers also gets billed on kW, through demand charges (coming soon) keyed to the highest power peak recorded in a short interval (commonly 15 minutes) across the billing period. That is how a site can install fast chargers, sell a modest amount of energy, and still open an alarming invoice.
The formula never changes. What changes is which side of it someone has decided to charge you for.