When using temperature values in expressions, it is generally safer to use units of [K] only. When units are used that have an offset (for example, [C]), they are converted internally to [K]. For terms that have the dimensions of temperature, any unit conversion will include the offset between temperature scales. However, in all other cases the offset is ignored because this is usually the most appropriate behavior. You should therefore take care when specifying an expression involving non-unit powers of temperature. For example, each of the expressions below is equivalent:

Temperature = 30 [C]
Temperature = 303.15 [K]
Temperature = 0 [C] + 30 [K]
Temperature = 273.15 [K] + 30 [K]

These are only equivalent because all units are to the power of unity and units other than [K] appear no more than once in each expression. The following expression will not produce the expected result:

Temperature = 0 [C] + 30 [C]

This is equivalent to 576.30 [K] because each value is converted to [K] and then summed. The two expressions below are equivalent (as expected) because the offset in scales is ignored for units that are not temperatures:

Specific Heat = 4200 [J kg^-1 C^-1]
Specific Heat = 4200 [J kg^-1 K^-1]