Charging your iPhone from a MacBook: what it costs

Published 27 August 2026 · every number below comes from one measured night on one MacBook. One night is not a law of nature, and this page says so wherever it matters.

Plugging a phone into the laptop is the least dramatic thing you can do to a Mac, which is why nobody measures it. We did, by accident: a phone went into the USB-C port before bed and came out about forty-nine minutes later, and the Mac was asleep with the lid shut the whole time. The battery history had the receipt.

What it cost

So the sleeping Mac spent 18.87 W against its own usual 0.81 W in the same night, in the same sleep state, read by the same instrument. That is a factor of 23. It is the cleanest contrast we have ever found in our own data, and it comes from the dullest possible event.

Why the number is bigger than a phone battery

An iPhone battery holds somewhere between 12 and 17 Wh depending on the model. The Mac gave up 15.08 Wh, which sounds like it handed over exactly one phone charge. It did not, and the difference is worth understanding.

15.08 Wh is what left the pack, not what arrived at the phone. Inside that figure sit the Mac's own sleep baseline for those minutes, about 0.67 Wh at the rate measured on the same night, and the work of the dark wakes that a connected device causes. Conversion losses take their share too. The honest statement is that the pack paid 15.08 Wh; how much of it became charge in the phone, we did not measure and will not guess.

Is it bad for the phone?

No. The phone is charging from a regulated port, the same way it charges from a wall adapter, and it negotiates what it will accept. Nothing about this is unusual or unsafe for the phone.

The cost is entirely on the Mac's side, and it is real. Twenty-two points of laptop battery is not a rounding error: it is roughly a fifth of the machine, spent while you were asleep and not using it. If you have ever woken up to a MacBook that lost far more overnight than it should have, a phone left in the port is a candidate worth ruling out.

How fast does it charge?

We measured what the Mac spent, which is 18.87 W averaged over forty-eight minutes. That is not the same as the phone's charging rate, and we cannot turn one into the other without measuring the phone as well. Anyone who quotes you a single wattage for "charging from a MacBook" is quoting the port specification, not a measurement of your machine.

What a battery monitor can and cannot tell you

This is the part most pages would skip, so here it is plainly.

What is measurable: that the pack lost 15.08 Wh in a forty-eight minute window, that the average draw was 18.87 W against a 0.81 W baseline, and that a USB-C port event falls inside that window. All of that comes from the Mac's own counters and the system power log.

What is not: which device was in the port. macOS records that something was connected, not what. The reason we can write "iPhone" at the top of this page is that a human remembers plugging it in, not because an instrument identified it. WattMate will not tell you the name of the device after the fact, and any tool that claims to is either reading a live connection or guessing.

That distinction matters more than it looks. A port event named USB-C_plug also fires for the Mac's own power adapter. We shipped a label that read "Device on USB-C" and found it was wrong on thirteen episodes out of fifteen for exactly this reason. It is fixed now, and the lesson stayed: we say what was measured, and we leave the space empty when the answer is not knowable.

One night, one Mac

Everything here is a single episode on a single machine. It is enough to show that the cost is large, because a factor of twenty-three against a same-night baseline is not something noise produces. It is not enough to tell you what your Mac will spend, which depends on your phone, its state of charge, and the port. Treat the shape as real and the exact figures as one data point.

Questions people ask

Does charging a phone drain a MacBook overnight?

In the one night we measured, yes, and substantially: 22 points of the laptop's battery over about forty-eight minutes. It stops when the phone stops drawing, so a phone that finishes charging early costs less than one plugged in empty.

Should I stop doing it?

Only if you need the laptop's charge more than the phone's. There is no damage argument here in either direction. It is a straight trade of one battery for the other, at a worse exchange rate than a wall socket because the Mac pays conversion losses and wakes up to service the port.

Will the Mac keep charging the phone while it is asleep?

It did in our measurement. The lid was shut and the machine was in normal sleep for the whole window, and the pack drained anyway.

Can I see this on my own Mac?

The energy is visible in the battery history: a sleep stretch that costs many times the usual baseline. What you will not get from any tool, ours included, is the name of the device that caused it after the fact.