Apple 18W USB-C Power Adapter on a MacBook Air: Measured
Apple's discontinued 18W USB-C adapter measured on a MacBook Air M5: 16.7 W delivered from a two-profile menu, and the 0.22 amps that made it famous for failing to power a HomePod mini.
Apple’s 18W USB-C power adapter - long since replaced by the 20W model - tested against a MacBook Air M5 with a low battery, per the methodology.
The numbers
| Rated | 18 W |
| Negotiated | 18 W (9 V up to 2 A) |
| Live draw | 16.7 W (8.8 V at 1.9 A) |
| Protocol | USB PD 3.0 |
Profile menu: 15 W (5 V/3 A), 18 W (9 V/2 A - active). That’s the entire list.
Two profiles, and the top rail is 9 V. There’s no 15 V or 20 V rung - the rails laptops normally charge on. This is a phone charger, and the Mac charges from it quite happily anyway.
The 0.22 amps that made it famous
This adapter’s place in Apple trivia comes down to a rounding error’s worth of current.
When the HomePod mini launched, it asked for 9 V at 2.22 A - exactly 20 W. This adapter offers 9 V at 2 A. Right voltage rail, right protocol, 0.22 A short. Plug a HomePod mini into it and the speaker would glow orange and refuse to run.
About a month later, Apple relaxed the requirement to 18 W in a software update, and this adapter started working fine.
The bigger charger that couldn’t
The part worth remembering: Apple’s 29W adapter - larger, higher rated - couldn’t power a HomePod mini at all. It produces its 29 W from a 14.5 V rail, and the mini only accepts 9 V.
More watts, wrong rail, no deal. Wattage alone tells you very little; what matters is whether the charger offers a voltage the device will accept. That’s the whole reason reading the profile menu beats reading the label.
No longer sold
Apple replaced this adapter with the 20W model on 13 October 2020, announced alongside the iPhone 12. The 20W took over at a lower price - $19 against the 18W’s $29 - and the 18W was retired.
The 20W is the direct successor, but it shares this adapter’s ceiling: a 9 V top rail and no laptop-oriented rung. For an Apple adapter with real headroom, the 40W Dynamic Power Adapter (60W Max) is the current choice.
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