AC or DC? It's the difference between an overnight top-up and a coffee-break rapid charge. Understanding why the two behave so differently is the key to charging faster and spending less. Here's how each works and how to use it well.
The core difference: where the conversion happens
Every EV battery stores and delivers direct current (DC), but the electricity in the grid, and in almost every home and business is alternating current (AC). Somewhere between the wall and the battery, that AC has to be converted to DC. The whole distinction between the two charging types comes down to one question: where does that conversion take place?
With AC charging, the current arrives at the car still as AC, and a device inside the vehicle, the onboard charger, does the conversion. With DC charging, the conversion happens inside the charging unit itself, which is far larger and more powerful than anything a car could carry, then feeds DC straight into the battery. That is why DC is so much faster: it sidesteps the car's built-in bottleneck.
Why the onboard charger limits AC speed
The onboard charger is a compact device built into the vehicle, and its size sets a hard ceiling on how quickly the car can accept AC power. Many are rated around 7 to 11 kW; some cars top out lower, and a few support up to 22 kW. This is why two cars plugged into the same AC unit can charge at different rates: the wallbox may deliver more, but the car only draws as much as its onboard charger can convert.
Manufacturers keep the onboard charger modest on purpose: a bigger converter means more weight, cost and heat, none of which earns its keep for charging that mostly happens overnight. So AC charging is deliberately gentle and capped, and no wallbox can push past the limit the car itself imposes.
AC charging: slow, inexpensive, everywhere
AC charging typically runs from around 3 kW on a basic home socket up to about 22 kW on a three-phase commercial unit. It is what you meet at home, at work and at destination chargers such as hotels, supermarkets and car parks. Because the hardware is simple and cheap to install, AC points are far more numerous than rapid ones, and often inexpensive or occasionally free.
The trade-off is time. AC charging is measured in hours, so it works best when the car was going to be stationary anyway. Left plugged in overnight, most EVs comfortably reach a full charge, and the slow, steady delivery is easy on the battery, part of why home charging suits most drivers day to day.
DC charging: fast, pricier, strategic
DC rapid and ultra-rapid charging skips the onboard charger entirely, so power levels jump dramatically. Rapid units commonly start around 50 kW, while ultra-rapid chargers reach 150, 350 kW and beyond. This is the road-trip tool: a well-matched car and charger can add a large chunk of range in the time it takes to have a coffee, often going from low to around 80 per cent in 15 to 40 minutes.
That speed comes at a price. DC electricity almost always costs more per kilowatt-hour than AC, and the fastest sites can carry an added premium. DC is best treated as a strategic tool for covering distance, not a daily habit, invaluable on long journeys, but an expensive way to do what a home wallbox does overnight for less.
The charging curve and why it tapers
DC charging does not deliver its headline power the whole time. It follows a charging curve: power builds early, holds near its peak while the battery is in a comfortable middle range, then tapers as the battery fills. The most efficient window sits roughly between 10 and 80 per cent, where the battery accepts current most readily.
Above about 80 per cent the battery management system deliberately slows the rate to protect the cells and manage heat. Those last points can take almost as long as the first eighty, which is why experienced drivers rarely wait for full at a rapid stop. Temperature matters too: a cold battery accepts power more slowly, so many EVs precondition the pack on the way to a charger.
The 80% rule. DC charging slows sharply above roughly 80 per cent to protect the battery. On a trip it is usually faster overall to stop at 80 per cent and drive on than to wait out the slow final stretch to full.
A real example: planning a longer drive
Imagine a weekend trip of a few hundred kilometres. You charge to full overnight at home on AC, which costs the least and is ready when you wake up. Partway along, with the battery low, you pull into a rapid site during a lunch stop. Because you arrive with plenty of room in the pack, the charger delivers strong power through its efficient window and brings you near 80 per cent within the break, so you drive on rather than waiting out the slow tail to full. AC handled the cheap charging; DC covered the distance when time mattered.
How to choose in the moment
- Overnight or a long daytime stop: AC. It is the cheapest option and you are not waiting on it.
- Mid-journey and you need range now: DC rapid, charging within the efficient 10 to 80 per cent band.
- Topping up while you shop or eat: whatever is available, even AC adds useful range over an hour.
- Daily driving from home: AC almost every time; save DC for trips where the extra cost buys time you genuinely need.
Matching the charger to your car
Your car has a maximum charging rate on each type, and it cannot exceed it. A 350 kW charger will not charge a car capped at 50 kW any faster than a 50 kW unit, and a 22 kW wallbox does nothing extra for a car whose onboard charger tops out at 7 kW. Knowing both limits tells you what to look for and stops you paying rapid-charger prices for speed the car cannot accept.
When you filter the map by power, aim for a charger that meets your car's maximum rather than the highest number on offer: matching the two is where real time and money are saved.
Ready to use it? Open the live map to find real chargers near you, check their current status, and add what you see for the next driver.