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The payback period is a lot faster than anyone’s suggesting, because no one’s taking into account the heat produced by incandescent bulbs, which requires costly air conditioning to remove.

Are you sure that it is significant enough to consider? But moreover what about heating savings in the winter? And wouldn't hot weather which needs AC be correlated with more sunshine and thus less need to have the light on...so perhaps the incandescent bulb is on more often when the building has the heating system going than when it has the ac going.