I have this Power Bank:
dodocool 20100mAh Externer Akku, USB C Powerbank, 45W Power Delivery Port, 2-Port 2,4 A Quick Charge, Tragbares Ladegerät mit Type C Kabel für iPhone, iPad, MacBook, Samsung und weitere
It is powerful enough to charge my XPS 9370 1 time completely.
Mind there is a recall for Dell hybrid chargers / power banks.
The offer of higher-power USB-C PD power banks is slowly improving. You should look for one supporting output power 45W at 20V (XPS 13 might work with less watts, but 20V is necessary and lower-powered ones need not support 20V; capacity around 20000mAh). Choose one which is claimed to work with XPS 13 by its vendor, or is at least reported to work by several users (Dell's USB-C PD implementation tends to be iffy, the PD power negotiation is sometimes reported to fail for no apparent reason).
USB-C PD power banks suitable for XPS 15 are still difficult to find. In a restricted mode, XPS 15 supposedly needs min 45W at 20V but most 3rd-parties fail; success is more likely with 60W at 20V still rare among power banks.