USB-C Power Output on M-series MacBook Pros

Source:https://chatgpt.com/share/67c0927e-1a40-8004-9916-9f57fbff0581

I'll find detailed information about the USB-C power output differences across the ports on MacBook Pro models with M-series processors. Some models may have variations in power delivery, especially when plugged in versus on battery. I'll let you know what I find soon.

USB-C/Thunderbolt Port Power Output on M-series MacBook Pros

Maximum Power Output per Port: Apple’s MacBook Pro models with M-series chips (which use Thunderbolt 3/4 over USB-C) can supply up to 15 watts (5 V/3 A) of power to bus-powered devices on a given port. This is the same limit as previous Thunderbolt 3 Intel modelswww.reddit.com. In practice, the ports operate at 5 Volts (the Mac doesn’t boost to higher USB-PD voltages for accessories), so 15 W is the max (5 V × 3 A). For example, a high-draw device like an iPhone or iPad will charge at roughly 10–12 W (around 5 V @ 2–2.4 A) when connected to a MacBook Pro – near the device’s fast-charge limitdiscussions.apple.com. The Mac’s 15 W capability is enough to quick-charge an iPhone (which typically pulls ~10 W) but won’t reach 20–27 W levels needed to fully fast-charge the latest iPhones or iPads at their maximum rate (the port stays around 15 W max)www.reddit.com.

All Ports Equal – Power Distribution for Multiple Devices: All USB-C ports on an M1/M2 MacBook Pro can provide the same peak wattage, but there is a total power distribution limit when you have many devices attached. Apple specifies that on MacBook Pros with multiple USB-C/TB ports, the system will allocate up to 15 W to the first devices that need it, and then lower power (7.5 W) to additional devices if neededapple.stackexchange.com. In other words, power is delivered on a first-come, first-served basisapple.stackexchange.com. For example:

  • 2-port MacBook Pro (13-inch, M1/M2): Can power one device up to 15 W and the second device up to 7.5 W simultaneouslyapple.stackexchange.com. (If only one device is connected, it can get the full 15 W from either port.)
  • 4-port equivalent (e.g. older Intel 15-inch): Could power two devices at 15 W each, and any others at 7.5 Wapple.stackexchange.com.
  • 3-port MacBook Pro (14/16-inch, M1 Pro/Max, M2 Pro/Max): Similarly, it can support two high-draw devices with ~15 W each, while a third device would be limited to ~7.5 W. (Apple didn’t explicitly publish a 3-port scenario, but it follows the same Thunderbolt/USB4 spec logicforums.macrumors.com.) Crucially, any individual port is capable of the full 15 W output – there are no “special” high-power vs low-power ports. It’s simply that the MacBook Pro manages the total budget if multiple peripherals draw power at once. This ensures, for instance, that plugging in two bus-powered SSDs or an iPad and another device can be supported (each getting full power if possible), but a third or fourth device may see reduced wattageapple.stackexchange.com. Apple’s documentation for Thunderbolt 3 noted, for a 15-inch MacBook Pro with four ports, it could supply two devices with 15 W and two more with 7.5 W, regardless of which side they’re onapple.stackexchange.com. The Thunderbolt/USB4 spec requires at least one 15 W-capable port, hence Thunderbolt 4 Macs continue to guarantee 15 W output (no increase over TB3’s 15 W)www.reddit.comforums.macrumors.com.

Power Delivery Prioritization: When multiple accessories are connected, the system prioritizes who gets full power in order of connection/request. The first high-draw device will get up to 15 W, and the next high-draw device can also get up to 15 W (on models with enough ports/controllers to support two) – after that, additional devices may be capped at ~7.5 W eachapple.stackexchange.com. Apple notes this “doesn’t matter which side of your MacBook Pro they’re connected to” – the allocation is dynamic and not fixed per port locationapple.stackexchange.com. In short, if you only have one device drawing power, it can use the full available 15 W on any port. With two devices, typically both can still get 15 W (especially on the 14/16″ models which have multiple Thunderbolt controllers). With three or more bus-powered devices, at least one will likely be limited to 7.5 W. If any device tries to draw beyond what the port can supply, macOS will throw a “USB Devices Disabled” warning or similar, and you’d need to use an externally-powered hub or connect that device to its own chargerforums.macrumors.comdiscussions.apple.com.

Real-World Findings (Intel vs M1 Generation): In real use, previous Intel-based MacBook Pros supplied about 2.1–2.4 A (≈10–12 W) to a single attached iOS device. For instance, a 2018 15″ MBP showed 500 mA base + 1900 mA extra = ~2400 mA available to an iPad (about 12 W at 5 V)discussions.apple.com. However, early users noticed the initial M1 Pro/Max 2021 models appeared to deliver less when on battery – one user found a 16″ M1 Pro could only provide ~1500 mA (1.5 A, ~7.5 W) to his iPad, which struggled to charge while in usediscussions.apple.com. By comparison, that same iPad would charge on the older Intel MBP (12 W) without issuediscussions.apple.comdiscussions.apple.com. This raised the question of a possible downgrade or change in behavior on Apple’s partdiscussions.apple.com. It’s unclear if this was a hardware design decision or a software limitation/bug (this was observed on macOS 12.0 in late 2021). Notably, Thunderbolt 4 certification still calls for 15 W output, and later observations suggest the M-series ports do support 15 W when conditions allow. In fact, a Belkin support document explicitly notes that MacBook Pro laptops support “more than 15 W” out of their USB-C ports when used as the host (i.e. they qualify as a 15W-capable source for devices)www.belkin.com. This implies that under the right circumstances the M1/M2 MacBook Pro can provide the full 15 W to peripherals (the Belkin adapter in question would shut down if the host only offered 7.5 W)www.belkin.com. So, the M-series models are not fundamentally limited to 7.5 W – they retain the 15 W capability, though Apple may manage it conservatively at times.

Behavior on Battery vs. AC Power: Apple hasn’t published an official statement saying “ports give X watts on battery vs Y when plugged in,” but evidence suggests the power source can affect output. To get full power output to USB devices, it helps if the MacBook Pro itself is connected to AC power. A top contributor on Apple’s forums pointed out that what the Mac is “willing to supply” might depend on whether it’s charging or running on batterydiscussions.apple.com. For instance, on battery alone the Mac might not drive a full 3 A to USB devices (to avoid rapid drain). The same expert noted the MBP tends to supply around 10 W (5.2 V @ ~2.1 A) by default to charge an iPhone – slightly below the theoretical 15 W maxdiscussions.apple.com. This aligns with Apple’s own default of ~2.1–2.4 A for device charging. If the Mac is plugged into its power adapter, it can likely deliver the higher end of that range without issue (since it’s not sacrificing its own battery). In short, when running on AC power, all ports can provide their full output (up to 15 W) to connected devicesdiscussions.apple.com. When on battery, the ports still work for charging accessories, but sustained high-power draw might be capped (around 7.5–12 W) to preserve the Mac’s battery. The user who saw only 7.5 W to his iPad was powering his MacBook from its battery at the time, which supports this theorydiscussions.apple.com. If you connect the MacBook to its charger, you may find the iPad or other device now draws closer to 2–3 A (restoring normal charge speed). Apple’s system will also automatically cut power to devices if your Mac’s own battery gets critically low.

Summary – Key Facts: All Thunderbolt/USB-C ports on M1/M2 MacBook Pros offer up to ~15 W of output for peripherals – there’s no difference in wattage from port to port. Any port can also be used to charge the MacBook itself, but only one power source is used at a time for charging the laptop (it will pick the highest-power adapter if you plug into multiple) – this doesn’t affect what the other ports do for accessoriesdiscussions.apple.com. When multiple devices are drawing power, the MacBook prioritizes the first one or two high-draw devices to supply at full 15 W, and will limit additional devices to 7.5 W each if neededapple.stackexchange.com. This first-come-first-served power distribution is managed automatically by macOS. Being plugged into AC generally allows the Mac to provide maximum power out to devices without draining its battery, whereas on battery power you may see the output curbed (some users measured ~7.5 W on battery vs ~12–15 W on AC)discussions.apple.comdiscussions.apple.com. These behaviors are backed by Apple’s documentation and testing: Apple officially stated the 15 W/7.5 W-per-port rules for MacBook Prosapple.stackexchange.com, and both user experiments and third-party accessory makers confirm that M-series MacBook Pro USB-C ports adhere to these power delivery characteristicsforums.macrumors.comwww.belkin.com.

Sources:

  • Apple Support document (via StackExchange) – Thunderbolt 3 ports power output specs for MacBook Proapple.stackexchange.com.
  • MacRumors Forums – Thunderbolt/USB4 spec requiring 15 W on first device, 7.5 W on additional devicesforums.macrumors.com.
  • Reddit r/MacBookPro – Confirmation that Thunderbolt 4 (M-series) keeps the same 15 W output as Thunderbolt 3www.reddit.com.
  • AppleInsider – Measurement of ~10 W (5.2 V @ 2.1 A) output to iPhone from MacBook Pro USB-Cdiscussions.apple.com.
  • Apple Community forums – User report of 2018 MBP supplying 2.4 A (12 W) vs 2021 M1 Pro MBP only 1.5 A (7.5 W) to an iPaddiscussions.apple.comdiscussions.apple.com.
  • Belkin support FAQ – Note that MacBook Pro (and iPad Pro) are 15W+ capable hosts for USB-C accessorieswww.belkin.com.
  • Apple Support article – “If you see USB Devices Disabled on your Mac” (power management for USB devices)discussions.apple.com.