For Simscape to be able to subtract two units, the units have to be able to be converted into similar units. For example, you can subtract Celsius from Kelvin because they are both temperatures, and you can convert Celsius into Kelvin (or Kelvin into Celsius).
In Simscape, 'C' represents Coulomb. Because the two units Coulomb and Kelvin cannot be converted into the same unit (they're not both temperatures), Simscape throws the error. Instead, represent Celsius with the unit 'degC'. The list of unit definitions for Simscape can be found at the following link:
