Since the power specification for USB 3 is significantly higher than that for USB 2, I'm wondering if it's important to choose a powered USB 3 hub over a less-expensive unpowered USB 3 hub.
I would strongly advise not to use any unpowered USB hubs (officially called "bus-powered hubs", and sometimes advertised as "mobile hubs") if you don't want communication/connect surprises, for the following reasons:
- A bus-powered (aka "unpowered") USB hub cannot officially supply full power to downstream ports, because the power source is already limited to one-port standard USB capability, minus the hub own IC power consumption (which is about 1W or 200 mA in today’s technology). This leaves about 700 mA to all downstream ports. Since the system must reserve 150mA minimum to each port, it leaves only 250 mA maximum for a 4-port hub, see Sec. 10-14 p.10-61. This power budgeting is new and improved over USB 2.0 specifications, where 100 mA was hard limit, no balancing.
- In theory, a bus-powered hub must advertise its power source in hub configuration descriptor. USB system software will then apply power policy and refuse to connect any USB device when the power budget is depleted. This is very inconvenient for ordinary users, so unscrupulous manufacturers are cheating. They lie in hub descriptor by setting it as "self-powered" (i.e. using external power source) no matter what. Since these “mobile hubs” cannot be USB-IF certified at all, they usually have several other out-of-control deficiencies, and who knows what else.
- One fundamental deficiency is that these cheap hubs don’t implement VBUS power off function. Even if all ports can be ganged and presumed to serve as “charging ports”, the VBUS toggle function should be implemented to signal to downstream devices when upstream hub connection to host starts running. See Sec.10.3.1.1 of USB 3.1 specifications.
- The other typical issue with cheap hubs is huge excess of capacitance on VBUS. The USB requires no more than 10 uF of load to upstream VBUS. Most cheap hubs use a common VBUS rail, but connect huge caps to each port. For example, the TP-LINK “mobile hub” Model UH400 has a 220uF cap per port, making the upstream VBUS loaded with lumped cap of 880uF, exceeding specifications by a factor of 88X. This cap can easily break any PC on connect, by nearly shorting +5V standby rail.
The bus-powered cheaply-implemented USB hubs are mostly good only for connecting mice and keyboards, and maybe one cheap pen drive. The newer Type-C connector and PD (power delivery) specifications might change this power balance picture, but the main USB specifications are not aware of this, and system software guidance is lacking for this case. And Type-C with PD won't be cheap by any means. Having a self-powered USB hub removes a lot of headache from USB interoperability standpoint.
Most USB hubs can just draw current from the computer itself. Keep in mind that a computer's USB 2.0 port can normally supply only 5V @ 500mA so if you try to draw more current than that you will probably need the power supply even if your hub is USB 3.0 .
If your computer is USB 3 ready you might get up to 900mA for data transfer and on dedicated charging ports you will get up to 3000mA with no data transfer.
It all depends on what you are connecting to it. If you are just using usb flash drives or DVD drives or mouse/keyboard you should be ok with unpowered.
But if you want stuff to charge from it with any speed (like a phone etc) or if you want to connect 3.5" hard drives or other higher power demanding peripherals to work you need a powered hub.