Charging a MacBook Air From Another MacBook Air: Measured

An M1 MacBook Air used as the charger for an M5 MacBook Air: it advertises a single 15 W profile at 5 V, and delivers 13.1 W. The first time in this series a Mac negotiated a PD contract on the 5 V rung.

An M1 MacBook Air used as the power source for an M5 MacBook Air, per the methodology. Not a charger at all, but a laptop handing power out of its own battery - and the profile it offers is unlike anything else measured here.

The numbers

SourceMacBook Air (M1), USB-C port
Negotiated15 W (5 V up to 3 A)
Live draw13.1 W (4.5 V at 2.91 A)
ProtocolUSB PD 3.0
Identifies asApple - shown as “Apple 15W Charger”

Profile menu: one entry. 15 W (5 V up to 3 A). That is the complete list.

The edge case: a PD contract at 5 V

Every wall charger in this series climbs a ladder and settles somewhere near the top - 12 V, 15 V, or 20 V. The 5 V rung exists on nearly all of them, but nothing ever stops there. It is the entry-level rail, the one used for a handshake before the real negotiation begins.

Here it is the only rail on offer, so the contract lands on it. This is the first negotiated Power Delivery contract in this series to settle at 5 V.

Worth being precise about what is unusual. Plenty of sources measured here deliver 5 V - every USB-A brick does. But those are not negotiating anything; they fall back to BC 1.2, a 2010-era charging standard with no profile list at all. This is different: a full PD 3.0 exchange, properly negotiated, that simply has one rung to choose from.

Why a Mac only offers one rung

A Mac’s USB-C ports can act as a power source, but they are host ports rather than chargers. They provide the standard 5 V bus rail and nothing above it. Generating 9, 15 or 20 V would require a boost converter dedicated to giving energy away, which is not something a laptop is built to do - the power is coming out of its own battery.

So the ceiling is 5 V times 3 A, or 15 W. Measured live it delivered 13.1 W, the voltage sagging to 4.5 V under a 2.91 A pull.

What it is good for

Emergencies, and very patient ones. 13 W will slow the drain on a laptop being actively used, or gain slowly on a sleeping one, while steadily emptying the donor machine. It is nothing like plugging into a wall.

The useful thing is knowing it works at all. If you are carrying two USB-C laptops and one is dying, the other is a 15 W power bank you already own.