The question as I understood it was how to make a device if output voltages make the conected device (assume resitive load) consume the same amount of wattage whatever resistance it might have.
Answer to this is that you can't easilly solve this with just a voltage or current regultor. It must me more "inteligent" than that. But interresting problem, I'll do a search on net
Cheers
[edit]
Sorry, no relevant hits for google search on "constant wattage output"