ATLAS · FIELD GUIDE
EV Fast Charging, Explained: What the kW Numbers Really Mean
A charger says 350 kW but your car charges at 150. Is the charger lying, is your car too slow, or is something else going on?
You pull into a charging stop, see a big number on the unit — 150 kW, maybe 350 kW — and plug in. Ten minutes later your car says it's charging at 90 kW, and the rate keeps drifting down. Nothing is broken. To read the fast-charging map, and to know which sites are actually worth stopping at, it helps to understand what that number means and why your car almost never matches it.
Kilowatts are a speed, not a size
The most common mix-up in EV charging is between two similar-sounding units.
A kilowatt-hour (kWh) is a quantity of energy — it's how big the battery is, like the size of a fuel tank. A kilowatt (kW) is a rate — how fast energy flows in, like how wide the fuel hose is. The charger's headline figure is always the rate.
Here's the rule that makes it intuitive: a charger holding a steady rate of X kW pours in about X kilowatt-hours every hour. A 50 kW charger adds roughly 50 kWh in an hour; a 150 kW charger adds about 150 kWh in the same hour — three times the range for the same wait. Since a typical electric car covers somewhere between 3 and 5 miles per kWh, that hour at 150 kW is very roughly 450 to 750 miles of range, and at 50 kW closer to 150 to 250. This is the whole reason the map colours sites by power: the kW number is the best single predictor of how long you'll stand around.
Why your car won't pull the full number
Two things sit between the charger's headline figure and the rate you actually get.
The car has its own ceiling. Every model has a maximum charging rate baked into its battery and electronics. If your car peaks at 150 kW, a 350 kW charger cannot force more into it — the charger only ever supplies what the car asks for. So a faster charger does nothing for a car that can't accept the speed. The rate you get is simply whichever is lower: the car's limit, or the charger's.
The rate falls as the battery fills. This is the part that surprises people. A battery charges fastest when it's fairly empty, and the car deliberately tapers the rate as it fills, to keep the cells cool and healthy. Plot the rate over a session and you don't get a flat line — you get a charging curve that starts high and steps down, often dropping sharply past 70–80%. The charger's advertised power is the peak of that curve under ideal conditions, not the average you'll see.
So a 350 kW charger and a car that peaks at 150 kW, charging from half-full on a cold day, might settle at 110 kW. The charger isn't lying and the car isn't faulty — you're just seeing the lower of two limits, partway down the curve.
AC vs DC: where the conversion happens
Batteries store direct current (DC). The grid delivers alternating current (AC). Something has to convert between them, and where that conversion happens is the entire difference between slow and fast charging.
- AC charging (home chargers, most destination chargers at hotels and car parks) sends AC to the car, where a small built-in converter turns it into DC. That onboard converter is deliberately compact, which caps most AC charging at around 7 to 22 kW — perfect for refilling overnight, far too slow for a road trip.
- DC fast charging does the bulky conversion inside the charging unit itself, then sends DC straight to the battery, skipping the car's little converter entirely. Freed from that bottleneck, DC chargers reach 50 kW and climb to 350 kW or more.
That's the line this map draws: it plots DC fast-charging sites only, from the 50 kW floor upward — the chargers built for getting back on the road, not for sitting overnight.
What this means for a real stop
Put it together and the practical advice falls out on its own. On a long drive, you want to arrive at a fast charger with the battery fairly low, charge through the quick middle of the curve, and drive on around 80% rather than waiting for full — because those last percentages crawl in at a fraction of the peak rate. A site's headline kW tells you how good its best case is; your car's ceiling and your starting charge decide how much of that you'll actually see.
When you read the map, the colour is a genuine signal — a 350 kW (amber) site really can refill a capable car far faster than a 50 kW (green) one — but it's the ceiling of what's possible there, shared between every car that's plugged in, not a guarantee for any single visit.
Frequently asked questions
What does kW mean on an EV charger?
A kilowatt (kW) is a rate — how fast energy flows into the battery, not how much it holds. Battery capacity is measured in kilowatt-hours (kWh); charging power is measured in kW. A rough rule: a charger delivering a steady X kW adds about X kilowatt-hours every hour. So a 50 kW charger adds roughly 50 kWh in an hour, while a 150 kW charger adds three times that in the same time. Most electric cars travel somewhere between 3 and 5 miles per kWh, so 50 kWh is very roughly 150–250 miles of range — which is why the kW figure is the single biggest factor in how long you wait.
Why doesn't my car charge at the charger's full advertised power?
Two reasons, and both are normal. First, every car has its own maximum charging rate, set by its battery and onboard electronics — plug a car that peaks at 150 kW into a 350 kW charger and it will draw at most 150 kW, because the charger can never push more than the car will accept. Second, the rate changes constantly as the battery fills: cars charge fastest when the battery is fairly empty and deliberately slow down as it approaches full, to protect the cells. The advertised number is the charger's ceiling, not a promise — the actual rate is whatever is lower, the car's limit or the charger's, and it falls as you fill up.
What's the difference between AC and DC charging?
It's about where the conversion happens. Batteries store direct current (DC), but the grid supplies alternating current (AC). Slow home and destination chargers send AC to the car, and a relatively small converter inside the car turns it into DC — which caps most home charging at around 7–22 kW, fine overnight but slow on a trip. DC fast chargers do the heavy conversion inside the charging unit itself, sending DC straight to the battery and bypassing the car's small onboard converter entirely. That's how they reach 50 kW and far beyond — and it's why the fast-charging map only plots DC sites.
Why does charging slow down after 80%?
To protect the battery. As lithium cells fill, pushing current in gets harder and generates more heat and stress, so the car's battery-management system steadily tapers the rate — often dropping sharply somewhere past 70–80%. The last 20% can take as long as the first 80%. That's why long-distance EV drivers usually charge from low to around 80% and then drive on: those middle percentages come in fastest, and waiting for a full battery at a fast charger is usually the slowest, least useful part of the stop.
SEE IT ON THE MAP
Everything in this guide is on the live Atlas map.