People confuse kW and kWh constantly. The terms sound nearly identical, both have “kilo” in them, and the only difference is one extra letter. But the distinction is the difference between asking “how fast is the water flowing?” and “how much water is in the tank?”
Get the distinction wrong and you’ll misinterpret every EV spec sheet you read.
The water analogy that actually works
Imagine filling a tank with water from a hose:
- kW (kilowatt) is the flow rate — how fast water is coming out of the hose. Bigger hose = higher kW.
- kWh (kilowatt-hour) is the total amount of water delivered — what ends up in the tank.
Multiply flow rate by time, and you get total amount. Same for electricity:
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). Different rates, similar 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”)
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 difference 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.
The charging time calculator handles this — 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 is longer because of three factors:
1. The battery doesn’t accept full power at high SOC
Above ~80% state-of-charge, every modern EV’s battery management system reduces the requested kW. By 95%, the rate might be a third of peak. This protects the cells.
So on a DC fast charger, the second half of the session is much slower than the first. The 80%-to-100% portion can take longer than 20%-to-80%.
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 can’t use it.
For DC fast charging, the same logic applies but on the DC side: a car capped at 100 kW DC will get 100 kW from a 350 kW station, not 350.
3. Temperature changes everything
Cold batteries charge slower. Many EVs precondition the battery before navigating to a DC fast charger to mitigate this, but in winter you’ll often see lower-than-expected rates anyway.
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.
“75 kWh battery” isn’t usable capacity unless specified. Most manufacturers quote usable capacity now, but some quote total. Tesla quotes usable; older Nissan Leafs quote total. A 24 kWh Leaf had only ~22 kWh usable.
Electricity rates on your home bill are in kWh. When you compare home charging to public charging, compare $/kWh both ways. Don’t compare $/kWh of home electricity to $/minute of DC fast charging — different units.
How efficient is your EV?
EV efficiency is reported in miles per kWh (or kWh per 100 miles, in some regions).
| Efficiency | Examples | Means |
|---|---|---|
| 5+ mi/kWh | Lucid Air (5 mi/kWh tested), best-case Tesla Model 3 LR | Very efficient — small battery goes far |
| 3.5-4.5 mi/kWh | Hyundai Ioniq 5, Kia EV6, most mid-range EVs | Typical |
| 2.5-3.5 mi/kWh | Larger SUVs (Rivian R1S, F-150 Lightning), high-performance trims | Energy-hungry |
| <2.5 mi/kWh | Heavy towing, high-speed highway driving | Worst case |
Multiply efficiency by battery size to get range. A 75 kWh battery × 3.5 mi/kWh = 262 miles. A 75 kWh battery × 2.7 mi/kWh (e.g. winter highway) = 202 miles. Same battery, different range.
The cost reframe
People often ask “is EV charging cheaper than gas?” The answer requires translating both to the same units:
- Gasoline car: gallons/100 mi × price per gallon = $/100 mi
- EV: kWh/100 mi × price per kWh = $/100 mi
Example: a 28 MPG car at $3.50/gallon → (100/28) × 3.50 = $12.50/100 mi. An EV at 3.5 mi/kWh and $0.15/kWh → (100/3.5) × 0.15 = $4.30/100 mi.
That’s a 3× cost advantage at home rates. Even at public DC fast charging ($0.40/kWh), it’s $11.40/100 mi — still slightly cheaper than gas. The cost calculator does this math.
Three things to take from this
-
kW = rate, kWh = total. Same relationship as miles-per-hour vs miles. One is speed, the other is amount.
-
Time to charge = kWh needed ÷ kW delivered. All charging-time math comes from this. The complications (taper, vehicle cap, temperature) bend the answer but don’t break the framework.
-
Compare apples to apples. When evaluating chargers, batteries, or costs, make sure you’re comparing the same unit. $/kWh both sides, miles/kWh both EVs, kW both stations.
Once these click, the rest of EV charging gets noticeably easier.