EV charging speed explained: kW, amps and km/h

Power is just volts times amps

Electrical power, measured in watts, is voltage multiplied by current. In an Australian home the voltage is fixed at a nominal 230 V (AS 60038), so the only variable that matters is current — the amps.

Power (W) = Volts × Amps
230 V × 32 A = 7,360 W = 7.36 kW

That's it. That's where "7.4 kW" comes from. Every single-phase home charger figure you'll ever see is 230 multiplied by something.

Why bigger amps need bigger wire

Current is what makes conductors heat up. Doubling the amps roughly quadruples the heat generated in the cable, which is why a 32 A charger needs properly sized cable on a dedicated circuit, and why you can't simply run one off a power point. It's also why AS/NZS 3000 requires a dedicated circuit rated for 100% of the continuous load.

Three-phase and the mysterious √3

A three-phase supply delivers three alternating waveforms, each offset by a third of a cycle. You might expect three phases to give exactly three times the power — but they peak at different moments, so they don't simply add.

The correct formula uses the line-to-line voltage (400 V in Australia):

Power (W) = √3 × Volts(line-to-line) × Amps
1.732 × 400 V × 16 A = 11,085 W ≈ 11.1 kW
1.732 × 400 V × 32 A = 22,170 W ≈ 22.2 kW

So √3 (about 1.732) is the factor that accounts for the phase offset. It's why the numbers land on 11 and 22 rather than 22 and 44.

What the wall offers vs what the car takes

Everything above is what your charger can offer. Your car has the final say through its onboard charger, which has a hard kW ceiling. The real charging speed is the lower of the two, always.

This is the whole ballgame, and it's why we built a database of onboard charger ratings — because manufacturers put the DC fast-charging number in the brochure (350 kW!) and bury the AC number that actually governs your driveway.

Charging isn't free: the losses

Your battery stores DC. Your house supplies AC. The car's onboard charger converts one to the other, and that conversion generates heat. Some energy you pay for never reaches the battery.

A defensible working figure for AC home charging is 85–90% efficiency — so 10–15% is lost. Source It's worse at very low power, because the car's own electronics and thermal management consume a roughly fixed amount regardless of how fast you're charging. On a 10 A power point that overhead is a much bigger slice of a small pie.

We use 90% at normal charger power and 85% on a power point. It's why our kilometres-per-hour figures are slightly more conservative than some calculators, which quietly ignore losses entirely.

Turning kilowatts into kilometres

This is the number people actually want. It needs one more input: how much energy your car uses per kilometre, in watt-hours per kilometre (Wh/km).

Energy into battery per hour = kW × efficiency
Range per hour (km) = (kWh/h × 1000) ÷ consumption (Wh/km)

7.36 kW × 0.90 = 6.62 kWh into the battery each hour
(6.62 × 1000) ÷ 170 Wh/km = 39 km of range per hour

So a standard Australian 7.4 kW home charger adds roughly 39 km per hour to a typical mid-size EV. Over an eight-hour overnight window: about 310 km.

Source: AAA Real-World Testing Program, via RACV.

Why the granny charger is slower than it says

A 10 A power point should give 230 × 10 = 2.3 kW. In practice portable chargers sold in Australia self-limit to about 8 A, giving roughly 1.8 kW. A 15 A socket similarly derates to about 12 A, or 2.8 kW.

The reason: EV charging is a sustained multi-hour load, unlike a kettle or a toaster that cycles. General-purpose sockets and their contacts degrade under continuous full-rated draw, and heat builds up in ways they weren't designed for. Source

Worth being precise here, because plenty of sites get it wrong: this is manufacturer safety practice, not an Australian legal requirement. The "80% continuous load" rule people cite comes from the American NEC and has no equivalent in AS/NZS 3000. In fact the Australian standard goes the other way for dedicated circuits, requiring them to be rated for 100% of continuous demand.

DC fast charging is a different animal

Everything above concerns AC charging, which is what happens at home. DC fast chargers bypass the car's onboard charger entirely and feed DC straight to the battery — which is why a car with a feeble 6.6 kW onboard charger can still take 140 kW at a roadside charger.

So your car's DC number tells you nothing about your driveway, and its AC number tells you nothing about a road trip. They're separate systems, and conflating them is the most common mistake in EV charging.

Australia has settled on Type 2 (IEC 62196-2, often called Mennekes) for AC and CCS2 for DC. CHAdeMO is legacy — effectively only the older Nissan Leaf. Worth noting that CCS2's dominance here is industry convergence rather than a government mandate, unlike in Europe.

Put it together

Three numbers give you everything: your supply, your car's onboard charger, and your consumption. The calculator does all of this and shows every step of the working, so you can check it rather than trust it.

Open the charging calculator →