Device loads:

The dongle has no way to know how much power the Type-A device connected to it will request. Apple adapters use a proprietary Type-A 5.5w signal*. Other USB-IF compliant hubs may use CDP 7.5w signal.

For example, the adapter pictured above must Reserve 5.5w of power from the source adapter at all times to power the Type-A port.

The dongle may need to take a number of actions if the input is less than 8.15w — one example being a 2.5w (5v*500mA) A-to-C cable. It may need to disable the 5.5w Type-A port, use power from the host, or shut off the Type-A port entirely.*

*(The actual Apple hub derates differently than this. This is merely an example.)

Cable and connector losses:

Cables don’t have 0 resistance. The USB-C specification accounts for this by permitting cables up to a 750mV loss from VBUS to GND. The only thing that “doesn’t count” for derating is the original cable resistance.*

*(This is illustrated in diagram above. “Allowed cable IR drop” is blue .)

Losses are normally accounted for by the device getting ‘a little less voltage’ than expected. Device-side voltage at ‘5v’ may droop as low as 4.00v when loaded to 3a. In this naïve example the dongle has to account for it.*

*(Note I use a 300mV drop for the captive cable. Captive designs can design-in a specific cable loss, rather than assume 750mV worst-case drop from a “bring-your-own-cable” situation that occurs by having a receptacle.)

This is likely why most dongles you see use a short captive plug. Receptacle designs are entirely possible… but would require more math, derating, and result in slower apparent charging. Tricky, no?

Internal hub draw:

The final concern is how much the dongle itself consumes. If the dongle uses expensive buck-boost regulator ICs, this could be in the milliwatts. If it uses cheap LDO linear regulator ICs, this could be in the watts. And manufacturers love cutting “Cost Down” corners.

I’ve seen poorly architected dongles try advertising 5v/2.4a through a linear regulator, with 20v negotiated pass-through. This means (20–5v*2.4a=) 36w of heat dissapated through a chip! Needless to say, it didn’t last too long before letting out the magic smoke.