Hi Folke!
Thanks for reaching Microsoft Community. We're here to help and let's work on this together.
Regarding your query on how much power can the tablet output to another device, the USB-C port is only used for charging and not to charge/power another device.
To know more about tech specs on Surface Go 3, please see through this link Surface Go 3 - Most portable 2-in-1 tablet & laptop - Microsoft Surface.
Please let us know how it goes. We look forward to hearing from you.
Greetings
RodelioP.