Even if the California plant is only selling these light-bulbs in California (which is doubtful), California itself gets much of their energy supplied from other states. Thus, regulating wattage can be included in the authority to regulate interstate commerce.To regulate Commerce with foreign Nations, and among the several States, and with the Indian Tribes
The reasoning they utilize is irrelevant to the fact that the constitution does, indeed, grant the authority for such regulation.
Edit: Scratch that. I just realized this is a California-specific situation. There's no need for the constitution to grant California this authority ove rtheir own intra-state commerce.
A conservative should know that.