Yes. When a MacBook runs on its battery, a phone that charges from its port takes its power from that battery. In our test on battery, an iPhone drew about 9 W from a MacBook, which is about a sixth of a 13-inch MacBook Air’s battery in an hour. Here is what that costs, and when it is fine.
How much power a phone takes
Apple does not publish how much power each port of a MacBook gives to a device. We measured it on one MacBook on October 11 and 12, 2026, with the figures that the Mac’s battery controller reports.
| Test | What we measured |
|---|---|
| The Mac on battery, an iPhone on a USB-C port | The iPhone drew 8.6 W, as the average of one minute |
| The Mac on its 96 W adapter, an iPhone on a USB-C port | The iPhone drew 15.3 W |
| The Mac on its adapter, an iPhone drawing about 9 W | The whole Mac used 23.9 W. After we disconnected the iPhone, it used 11 to 13 W |
The last test shows that the Mac uses more power than the phone receives. A phone that took about 9 W added 11 to 13 W to the total of the Mac.
A phone takes the most power while its battery is low. The draw falls as the phone gets near full.
What it costs in battery time
| MacBook | Battery (Apple’s figure) | One hour of a phone at 8.6 W |
|---|---|---|
| MacBook Neo | 36.5 Wh | About 24% of the battery |
| MacBook Air 13-inch | 53.8 Wh | About 16% |
| MacBook Air 15-inch | 66.5 Wh | About 13% |
| MacBook Pro 14-inch | 72.4 Wh | About 12% |
| MacBook Pro 16-inch | 100 Wh | About 9% |
The table counts only the power that the phone receives. The Mac itself uses more than that to supply it, so the real cost is higher.
When it is fine
- The Mac is on its adapter. The adapter then powers the phone, the Mac and the battery. Check that the adapter has enough watts for all three. See MacBook plugged in but not charging.
- You need the phone more than the Mac. On a train, a quarter of the Mac’s battery can be a good trade for a charged phone.
When to use a wall charger instead
- The Mac is on battery and you need it for hours. A phone can take a sixth of an Air’s battery in an hour, and more when you count what the Mac uses to supply it.
- The Mac is going to be turned off. Apple says to keep the computer on. An iPhone connected to a computer that is turned off can drain instead of charge.
- You want speed. A wall adapter of 20 W or more can give the iPhone more than the 9 to 15 W we measured from a port on a MacBook.
macOS does not show the power to the ports
The battery menu, the Battery settings and Activity Monitor show nothing about the power that goes out of the ports. A phone that charges from the Mac makes the battery go down faster, and nothing in macOS tells you why.
BatteryBud is a battery monitor for Mac. From version 1.3.0 it has Power flow on MacBooks. The map has a Devices group with each device that charges from a port, in watts. A device has its own name when macOS reports one for that port, for example an iPhone or an Apple Watch charger. The map keeps the history too, so you can see how many watt-hours went to the phone this week.
BatteryBud can also show the iPhone itself, with its level, its battery health and its charge cycles, over Wi-Fi. See how to check your iPhone battery from a Mac. For the other causes of a fast drain, see what is draining your Mac battery.