The versatile USB-C port permits all types of knowledge and energy to go throughout it. You possibly can connect docks that allow you to increase your accessible ports, with spots for USB Kind-A, HDMI or DisplayPort video, ethernet, and passthrough energy through a USB-C energy cable.

However a dock that doesn’t have an exterior energy supply—plugged in through an adapter to an AC outlet or by way of USB-C energy pass-through—has strict limits on the ability it could draw from a Mac and allot to related drives and different peripherals. (USB-powered followers and tea heaters, maybe.)

You’ll find a superb rationalization of this on the product web page for Different World Computing’s USB-C Journey Dock ($54.99). This compact dock has two USB Kind-A (3.1 Gen 1) ports, pass-through USB-C energy (as much as 100 watts), HDMI (4K as much as 30Hz), and an SD Card slot (UHS-II).

You possibly can piece collectively some limits if you happen to dig into the dock’s tech specs and footnotes. When there’s no USB-C energy adapter plugged into its pass-through USB-C port, it could draw up 15W from the host Mac, consumes 1.5W for itself, and divvies up the remainder between the 2 Kind-A ports, the SD card slot, and the HDMI port. These latter two functions don’t require a lot, however it could matter with simply 12W to play with.

When plugged right into a USB-C energy adapter, nonetheless, the dock can reserve as much as 25.5W from as much as 100W of energy delivered by way of the adapter: 1.5W for itself and as much as 12W for every of the Kind-A ports. The rest is handed by way of to cost the pc.

An exterior USB 3.0 or later SSD or onerous disk drive has to work inside a peak energy price range of about 4W. The utmost they will count on to attract from a typical port is 4.5W (5 volts at 900 milliamps), the minimal port spec for USB 3. Staying at 4W or beneath avoids getting too near the utmost and inflicting the port to close down. (CD/DVD drives and bus-powered peripherals skirt the road and will cross it. Consequently, they usually come geared up with two USB plugs—I had a Fujitsu scanner that took that method—or an AC adapter to run the disc’s laser or different power-hungry components.)

OWC

When related on to a pc, a drive can virtually at all times pull that energy repeatedly, similar to you may plug in a cell system and cost at as much as 5W over a time period. A cell system doesn’t thoughts if the ability sags, however an exterior drive’s controller will crash or flip off, leading to an involuntarily unmounted quantity. You would possibly lose information or need to run Disk Utility to restore it.

With a dock that manages bus energy amongst a number of gadgets, you may run afoul of the bus-powered maximums. Suppose you’re utilizing the OWC dock above and also you’ve received an exterior monitor connected, an SD card inserted, and a tough drive related plugged into one port, and have a USB charging cable in your iPhone plugged into the opposite—properly, that could possibly be proper beneath 12W. Any hiccup or millisecond power-balancing situation, and the SD card and drive get knocked offline briefly.

Verify the numbers

My recommendation ought to make you cautious of any dock that doesn’t presents wattage numbers in its technical specs. One reader despatched us a message about a problem with a drive constantly disconnecting with a selected dock—one I additionally occur to personal and have seen comparable energy points with. (I don’t need to name out the corporate since this isn’t a evaluate.)

Whenever you seek the advice of the product web page for this dock, the corporate doesn’t checklist its most output, nor does it name out any port limitations—save one. It says you can’t connect an exterior CD/DVD reader (together with Apple’s outdated SuperDrive, a blast from the previous) as a result of the dock can’t ship these peripherals’ peak energy wants.

Nevertheless it makes me surprise in the event that they skimped on the general energy availability, making the dock considerably much less helpful with out clients realizing it.

Thunderbolt 3 and 4 docks don’t appear to have the identical drawback as USB 3-only USB-C docks as a result of bus-powered Thunderbolt docks appear fairly uncommon—I’ve solely discovered a couple of minimally featured Thunderbolt 3 docks of that sort. The upper energy calls for of Thunderbolt docks usually want wattage above what may be delivered from a USB-C port on a Mac laptop computer or desktop. The docks both require a high-wattage USB-C cable related to an adapter or embody an AC energy adapter.

This Mac 911 article is in response to a query submitted by Macworld reader Greg.

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