The unrealistic assumption of someone offering to pay the part of your electricity bill that comes from lighting, no matter how big? Ok, yes, that’s clearly unrealistic, but, the cost of this presumably wouldn’t be like, totally insane, right?

Of course, I don’t mean that in realistic scenarios that the amount of energy used for lighting would decrease directly by the proportion given by the increase in efficiency of light-per-power, which would only happen if you had exactly no demand for more lighting than you have even if it was free, but that doesn’t mean the energy used would necessarily increase, just that it wouldn’t decrease as much as it would if you consumed the same amount of lighting.