I am currently looking at eGPUs for my laptop (HP Spectre x360 15"). Because the standard power brick is rated for 150W, the USB-C standard would only provide 2/3 of the required power, so even an eGPU that can deliver 100W over the USB-C/Thunderbolt connection would be inadequate.

One eGPU (the AkiTiO Node Pro) offers two USB-C/Thunderbolt ports that can supply 60W each. My laptop has two USB-C/Thunderbolt ports, both of which can be used for charging. If I were to connect both ports on the laptop and eGPU, could I deliver 120W to my laptop, or am I still limited to 100W?

I am guessing this depends on the number of USB controllers present since each should (likely) allow for 100W, so having two would mean a theoretical limit of 200W while one would limit me to 100W.

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  • Oh, reading more I see you're asking about powering the laptop from the eGPU. Specs for the laptop report it ships with a 90W adapter, not a 150W. I'm guessing your computer only requires the 90W, not 150W. – music2myear Jun 13 at 18:35
  • @music2myear The i7-8705G equipped model comes with a 150W power supply. – Hari Ganti Jun 13 at 21:22
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    "My laptop has two USB-C/Thunderbolt ports, both of which can be used for charging" Used for charging what? To charge your laptop, or to charge some other devices FROM your laptop? – Ali Chen Jun 13 at 22:44
  • @AliChen It's USB-C, so... both. In this case, I am talking about charging my laptop from the eGPU, but I could charge my phone from my computer with the same ports and the same cable – Hari Ganti Jun 14 at 16:52
  • "It's USB-C, so... both" This is not necessarily true. A Type-C can be "host only" and may be designed not to reverse its power role. Don't you find it strange that the native AC-DC "brick" is rated at "150" Watts while a Type-C port can't possibly exceed 100 W? If a laptop has a dedicated barrel-type power connector, it is unlikely that it can be charged over Type-C, too much work/design is needed. – Ali Chen Jun 14 at 19:25
up vote 2 down vote accepted

USB specifications for Type-C connectivity do not define any provisions to share or combine individual port's power. It certainly can be defined and designed in, but it will be a proprietary function, and generally you shouldn't expect power to be "combined". Feeding a load from multiple sources was always tricky and requires some "balancing", "equalization", or some other "sharing policy", which needs to be defined and specified. . It can work by accident (as various Y-cables do), but likely not. You should direct your question to the manufacturer of your laptop.

In theory, yes. However, doing this sanely would require balancing the load across the two USB-C connections would be absolutely necessary for this to work, but your laptop probably doesn't support doing this, and the eGPU almost certainly doesn't. Realistically, USB-PD compliant devices are designed to only use one USB-C connection, and if they need more than 100W, they use some external PSU. Some PSU's can source more than this, but they use special cabling for it, and may not be completely USB-PD compliant).

Now that said, using an underpowered power supply is almost guaranteed to not damage your system. It will either charge more slowly than normally, or discharge more slowly than normal, but for any modern system it shouldn't damage anything.

  • I will assume that the battery management unit will handle everything between the battery and the USB controller, but really it's a matter of whether the USB spec only allows for 100W per device or per controller. Also, the eGPU should balance fine since each USB-C port is rated for 60W, independent of the other. – Hari Ganti Jun 13 at 21:25

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