Only 15W. XPS 13 has a 45W adapter so if the laptop is off it will take triple the time to charge roughly and if on will hardly charge it at all. If you were fully loading it up, it might not even be able to sustain it. Should be pretty easy to get an idea with a Kill-A-Watt type meter on the input side to the existing supply.

The Dell TB15 has a much higher power output and is a full replacement for a power supply. Unfortunately the rest of it seems badly supported by drivers/firmware/buggy hardware.

Lots of good stuff from Mark but yeah it's pretty confusing for technical people. How is it not going to be confusing for those that aren't so technical.