A kilowatt (kW) is a rate: how fast electricity is flowing right now. A kilowatt-hour (kWh) is an amount: how much energy has flowed in total. Chargers are rated in kW (speed); batteries and bills are measured in kWh (quantity). Charging time is the kWh your car needs divided by the kW the charger and car can sustain.
Key Takeaways
- kW is speed, kWh is amount — chargers are rated in kW, batteries and bills in kWh.
- Charging time is energy needed (kWh) divided by power (kW), before losses and the car’s own limits.
- A charger’s kW is a ceiling; the car’s onboard charger often sets the real AC rate.
- Session cost depends on kWh used and your per-kWh price, not on how fast the energy arrived.
Two Units, Two Different Questions
The two units look alike on a spec sheet, which is why they get mixed up. They answer different questions:
| Unit | What it measures | Where you see it |
|---|---|---|
| Kilowatt (kW) | Power — the rate of energy flow | Charger ratings, your car’s onboard charger limit, the live number in a charging app |
| Kilowatt-hour (kWh) | Energy — a total amount delivered or stored | Battery size, energy added per session, your home electricity bill |
Put simply: 1 kW flowing for one hour delivers 1 kWh, so a 7 kW charger adds about 7 kWh per hour, before losses.
The One Formula Behind Every Charging Time
Time = energy ÷ power: 10 kWh needed at a steady 10 kW takes about one hour. The U.S. Department of Transportation puts Level 1 at about 1 kW (2–5 miles of range per hour), Level 2 at 7–19 kW (10–20 miles per hour), and DC fast charging at 50–350 kW. Apply the formula to a 10 kWh top-up:
- Level 1 at about 1 kW: roughly 10 hours for 10 kWh — an overnight top-up, not a refill.
- Level 2 at 7 kW: roughly an hour and a half for 10 kWh.
- DC fast at 100 kW: about 6 minutes for 10 kWh, where the car can accept that rate.
Real sessions run a little longer than the raw division, because of losses and the car’s own rate management. Our guide to how many kWh it takes to charge an EV works the same maths for full charges.
Why a High-kW Charger Does Not Always Charge Faster
The kW figure on a charger is a ceiling, not a promise. On AC charging, the car’s onboard charger does the converting, so the session runs at the lower of the two ratings: a 7 kW wallbox cannot push a car with a smaller onboard charger any faster than that car allows. Our breakdown of how many kW a Level 2 charger is covers the common ratings and ceilings.
The kWh Is Also the Unit on Your Bill
Your home electricity is priced by the kWh — U.S. average residential price 18.31 cents per kWh (EIA). Session cost is therefore about the amount, not the speed: 10 kWh costs about $1.83, overnight or in minutes. Two classic mistakes follow: expecting a bigger-kW charger to cut the energy bill (it changes time, not energy), and expecting a big battery to charge fast just because it is big (a bigger tank takes longer to fill at the same kW).
Reading Charger Ratings With the Two Units Straight
When you compare chargers, match the kW rating to your car and supply, and think in kWh for battery size and running cost. The EV-ACS4005T 40A NACS portable charger delivers 9.6 kW — roughly 9.6 kWh per hour plugged in, before losses. The EV-ACT3204PE 22 kW wallbox is a three-phase unit for cars and supplies that can use it; a car with a smaller onboard charger sets its own lower pace. The honest question is the same for both: how many kWh do you need overnight, and what kW can your car and supply sustain?
Frequently Asked Questions
Is a 7 kW charger the same as 7 kWh?
No. 7 kW is the rate. Leave it running for one hour at full rate and it delivers about 7 kWh of energy, slightly less after losses. The kW tells you the pace; the kWh tells you the total.
Which unit describes my car’s battery?
kWh. Battery capacity is an amount of stored energy, so a “60 kWh battery” is a size statement that says nothing by itself about charging speed.
Why does the kW in my app fall as the battery fills?
The car reduces the rate as the battery approaches full, to protect it. The kW falls; the kWh total keeps adding up until the session ends.
Related Reading
- How Many kWh Does It Take to Charge an EV?
- How Many kW Is a Level 2 Charger?
- Level 1 vs Level 2 vs DC Fast Charging: A Practical Comparison
- How Do Home EV Chargers Work?
