kW vs kWh: The One Distinction That Explains Everything Else
Confusing power with energy is why charging specs feel impenetrable. Learn this once and charge times, electricity bills and charger specs all become arithmetic you can do.
Almost every confusing thing about EV charging traces back to one conflation: treating kilowatts and kilowatt-hours as if they were the same unit. They are as different as miles per hour and miles.
Get this straight once and charging specs stop being jargon.
The analogy that actually works
Think about filling a bucket from a tap.
- kW (kilowatts) is the flow rate. How fast water leaves the tap. Open it wider, higher kW.
- kWh (kilowatt-hours) is the volume. How much water ended up in the bucket.
A tap running at a given rate for a given time delivers a quantity:
kW × hours = kWhThat is the entire relationship. Everything else follows from it.
Which unit describes what
| Thing | Unit | Why |
|---|---|---|
| Battery capacity | kWh | a quantity stored |
| Charger output | kW | a rate of delivery |
| Your electricity bill | kWh | quantity consumed |
| Electricity price | ¢ per kWh | price of a quantity |
| Car efficiency | miles per kWh | distance from a quantity |
| Onboard charger limit | kW | maximum rate the car accepts |
If you ever see a battery quoted in kW, or a charger in kWh, someone has made an error — and it is worth being sceptical of the rest of that source.
Doing the arithmetic
How long will charging take?
hours = energy needed (kWh) ÷ power (kW)Adding 40 kWh with a 7.7 kW charger: 40 ÷ 7.7 = 5.2 hours. Add ten to fifteen percent for charging losses and call it six.
What will it cost?
cost = energy (kWh) × rate (¢/kWh)Note that power does not appear. A faster charger does not cost more per mile. You are buying a quantity, not a rate.
What does a full charge take?
A 75 kWh pack from near-empty, at three common rates:
| Power | Time |
|---|---|
| 1.4 kW (Level 1) | ~54 hours |
| 7.7 kW (32 A Level 2) | ~10 hours |
| 11.5 kW (48 A Level 2) | ~6.5 hours |
Which is why Level 1 works fine for topping up daily driving and badly for recovering from a long trip — the rate is the constraint, not the total.
The bottleneck nobody mentions
Here is where the distinction earns its keep.
Your home charging speed is the lowest of three limits:
- The circuit. A 40 A circuit allows 32 A continuous — 7.7 kW at 240 V.
- The charger. Whatever it is rated and configured for.
- The car’s onboard charger. The converter inside the vehicle.
That third one catches people out constantly. Plenty of EVs have onboard chargers limited to 7.2 or 7.7 kW. Fit an 11.5 kW wall box and a 60 A circuit to one of those and it will still charge at 7.7 kW, because the car cannot accept more.
Before paying for a 48 A charger and the heavier circuit it needs, look up your car’s onboard AC charging limit. If it is 7.7 kW, the upgrade buys you nothing at all.
Why DC fast charging is quoted differently
Public fast chargers advertise in kW — 150 kW, 350 kW — and those numbers are peak figures the car will hold only briefly, usually at a low state of charge.
The battery’s acceptance rate falls as it fills. That is why fast-charge sessions are quoted 10% to 80%: the last fifth takes disproportionately long, because the rate collapses even though the remaining quantity is small.
Same distinction. Peak kW sells the charger; delivered kWh over the session is what you actually got.
The one-line version
kW is how fast. kWh is how much. You buy kWh. You wait on kW.
Once that is automatic, the rest of home charging is arithmetic — including what it costs, which is just your rate multiplied by the quantity you use.
Frequently asked questions
What is the difference between kW and kWh?
kW is a rate — how fast energy moves right now. kWh is a quantity — how much energy moved in total. A 7 kW charger running for 3 hours delivers 21 kWh. Power is the tap; energy is what ends up in the bucket.
Is a bigger battery measured in kW or kWh?
kWh. Battery capacity is a quantity of stored energy, so a 75 kWh pack holds 75 kilowatt-hours. If a spec sheet gives a battery in kW, it is wrong.
How do I work out charging time?
Divide the energy you need by the power you can deliver. Adding 40 kWh at 7.7 kW takes about 5.2 hours before losses, or nearer 6 with them.
Which number does my electricity bill use?
kWh. You are billed for quantity consumed, not for the rate at which you consumed it — though some utilities add demand charges for commercial customers based on peak kW.
Does a higher kW charger cost more to run?
No. Delivering 40 kWh costs the same whether it took four hours or eight, because you are billed on energy. The exception is time-of-use tariffs, where charging faster inside a cheap window can genuinely save money.
