Hi,
I understand that you want to connect to subsystems, and you are facing an issue with units.
Different sensors can be used to match units on the Hydrogen tank and Fuel cell sides. Currently a Flow Rate sensor is used which outputs kg/s. If kg/s unit is to be retained, the input on the fuel cell side will have to be changed. Whereas, if Pa is required, a Pressure & Temperature sensor can be used instead.
To view the unit output by the sensors, enable Units from Information Overlays, which can be found in the Debug tab.
Here are different ways to connect the Hydrogen tank: