Quick Answer: A charging hub divides one supply, so the number on the adapter is not the number any single port receives. Budget 8 to 12 W of overhead for the hub itself, then share the remainder. A 100 W supply leaves about 88 W, which is one 65 W laptop or four ports at 22 W each. Port current never exceeds 5 A at 20 V on USB PD 3.0, and a 240 W PD 3.1 port needs 48 V and an EPR-rated cable.
Four devices, one adapter, and one of them charges at half speed. That is not a fault. A hub allocates a fixed power pool, and every port negotiates from the same pool in the order devices are connected, so the first device to handshake usually takes the largest share.
Where the budget goes
Hub overhead
A powered hub with Ethernet, card readers and video conversion consumes 8 to 12 W before any device charges. That power comes from the same supply as the ports, so a 65 W adapter on a feature-rich hub delivers closer to 53 W to the laptop.
Passthrough versus per-port allocation
A passthrough hub gives one designated port almost everything the supply provides and leaves 5 V 0.9 A for the rest. A per-port hub splits the pool evenly, which is better for phones and worse for a laptop. Read the port table on the unit, not the headline wattage.
The numbers per port
The baseline for a downstream port is 5 V at 900 mA, which is 4.5 W. Battery charging at 1.2 pushes 5 V at 1.5 A, or 7.5 W. USB PD goes further: 5 V 3 A, 9 V 3 A, 15 V 3 A and 20 V 5 A give 15 W, 27 W, 45 W and 100 W, and PD 3.1 extended power range adds 28 V, 36 V and 48 V at 5 A for 140 W, 180 W and 240 W. A usb hub that advertises 100 W passthrough is quoting the input capability, and 10 to 15 W of it disappears inside the hub.
| Supply | Usable after hub | Largest single port | Four ports, even split |
|---|
| 45 W | 37 W | 35 W | 9 W |
| 65 W | 57 W | 45 W | 14 W |
| 100 W | 88 W | 65 W | 22 W |
| 140 W PD 3.1 | 125 W | 100 W | 31 W |
| 240 W PD 3.1 | 220 W | 140 W | 55 W |
Heat and cable losses
A hub converting 100 W at 88 percent efficiency dissipates 12 W inside a sealed case with no fan, which is why a plastic body reaches 50 C on the top surface and throttles its own ports. Cable loss is the second term: 5 A through 28 AWG conductors drops about 0.4 V per metre of pair, so a 6 ft (1.8 m) thin cable loses 2 W and runs warm. Use 20 AWG or a cable with an E-marker chip above 3 A, since the chip is what tells the hub the cable can carry 5 A.
When the hub is the wrong device
A hub shares one host connection and one power pool. It cannot charge two laptops at full rate, and it will not charge a device that needs a proprietary fast-charge protocol it does not implement. Where the requirement is to move data between machines rather than share accessories, the correct device is a switch or a KVM, and the distinction is covered in hub vs switch vs router. For a single-cable desk with display, Ethernet and charging on one connector, a type c hub with a 100 W input and a documented 85 W passthrough is the realistic minimum.
Practical scenarios
- Situation: A 65 W ultrabook and three phones share one hub on a 100 W supply. The laptop takes 65 W and the phones fall to 5 V 0.9 A, so a phone that needs 18 W charges in about 3 hours instead of 1.5.
- Situation: A 50 Wh tablet battery charged at 22 W from 20 percent takes roughly 90 minutes; the same battery at 9 W takes over 4 hours, which is the practical cost of an even four-port split.
- Situation: A wall-mounted hub runs warm and drops ports after an hour. Move it off the insulation in the wall cavity and leave 2 in (50 mm) of air, because the case is dissipating 12 W with no convection path.
- Situation: A 140 W laptop charger does not fast-charge through a 3 ft extension cable. The cable has no E-marker, so the hub limits the port to 3 A and the laptop settles at 60 W.
FAQ
Does a 100 W hub charge a 100 W laptop at 100 W?
No. The hub consumes 10 to 15 W for its own circuitry and the cable loses another 1 to 2 W, so a 100 W input delivers about 85 W at the laptop. Size the supply 20 percent above the laptop rating.
Why do ports stop charging when a laptop is connected?
The pool is shared and the laptop negotiated the largest profile. Ports that cannot be served fall back to 5 V or suspend charging. A hub with a separate always-on charging port avoids the behaviour.
Is a charging hub safe to leave connected?
Yes, if it is a listed product with over-current and over-temperature protection. Check that the case stays below 60 C in still air, and avoid daisy-chaining hubs, which multiplies both the overhead and the heat.
