Quick answer: A USB-C power hub takes one charger input and splits it between laptop charging and its own ports. The hub keeps a reserve, often 10 W to 15 W, so a 100 W charger may deliver 85 W to the laptop. Data and video share the same lanes, so a busy hub can drop video to 4K30 or push USB 3 traffic down to USB 2 speeds.
The specification on the box says 100 W pass through, and the laptop still reports a slow charger warning. That is not false advertising, it is arithmetic. A hub needs current for its controller, its card reader, its Ethernet chip and its video converter, and it takes that current from the same budget before passing the remainder upstream. Understanding the split prevents most of the disappointment around these devices.
How power delivery is split
Power Delivery negotiates a contract between the charger and the device. When a hub sits in the middle it becomes the contract partner: it asks the charger for a voltage and current, uses what it needs, and offers the rest to the laptop under a new contract. The reserve is not a fixed number across products, but 10 W to 15 W is typical for a hub with Ethernet, HDMI and a card reader. A hub with an internal SSD enclosure or a 2.5G Ethernet chip reserves more.
The practical consequence is that a 65 W charger behind a hub may only deliver 50 W, which is enough for an ultrabook but not for a workstation laptop under load. Match the charger to the laptop plus the reserve, not to the laptop alone.
Where the bandwidth goes
| Port combination | Lanes used | Result | Watch for |
|---|
| USB 3 plus 4K60 HDMI | Four DP lanes | Full video, USB at 5 Gbps | Rare outside premium hubs |
| USB 3 plus 4K60, shared | Two DP lanes | Video may fall to 4K30 | Most common design |
| Dual display plus USB 3 | Two DP lanes, MST | Lower refresh per screen | Check resolution pairs |
| Card reader plus Ethernet | Shared 5 Gbps bus | Slower transfers under load | Copy one task at a time |
Thermals and the aluminium case
Why the hub gets warm
Conversion losses and the Ethernet PHY produce heat, and a sealed aluminium shell has limited surface area. A hub that is warm to the touch is normal. A hub that becomes too hot to hold is dissipating more than its design allows, usually because the laptop is pulling full charging current while an external drive runs.
What heat does to the link
High temperature raises resistance and reduces timing margin inside the video converter. Symptoms appear as intermittent flicker or a dropped display after an hour of work, not at start up. Raising the hub off the desk or choosing a model with a metal base and vents is more effective than adding a fan.
Bus powered against self powered
A bus powered hub draws everything from the laptop port, which is fine for a mouse, a keyboard and a card reader. Add a portable SSD and the total can exceed what the port will supply, and the drive disconnects under load. A self powered hub with its own supply removes the problem but adds a second cable and a wall socket. For a fixed desk, self powered is the more reliable arrangement.
Practical scenarios
- Single cable desk: one USB-C connection for charging, display and peripherals works if the charger exceeds the laptop draw by at least 20 W.
- Field kit: a bus powered hub with HDMI and Ethernet is enough for presentations, and it stays inside the port power budget.
- Photo workflow: copy cards through a self powered hub so the reader and the SSD do not compete for the same current.
- Meeting room: fix the hub to the table and power it locally, so visiting laptops only need one cable.
FAQ
Why does my laptop say the charger is slow?
The hub reserved part of the budget, or the charger does not support the voltage the laptop requests. Check the reserve and the charger profile.
Can a power hub charge a phone and a laptop at once?
Only if the charger has enough headroom. The hub splits one input, it does not create additional power.
Does a hub reduce data speed?
All downstream ports share one upstream link, so simultaneous transfers are slower than a single direct connection.
Next step
Add up the laptop draw and the peripheral load before choosing the hub, then confirm the video mode. The USB-C docking station guide covers the fixed desk alternative, the USB-C video cable guide explains lane allocation, and the USB4 and Thunderbolt guide sets the bandwidth expectations for newer hosts.
