Taxing light bulbs
If fluorescent bulbs really are cheaper to operate, there's your "tax" already.
You generally don't need to force people into something that's a good idea--if it's cost-effective and works. For example, higher gas prices are all it takes to get people into high-mileage cars (the U.S. auto industry is in a jam partly because gasoline prices oscillate a lot faster than the lead time for producing a particular car model in large numbers).