But the power use is also constant over time t, such that any reduction in time use is effectively a constant drop in cost. You are literally shrinking either

But the power use is also constant over time t, such that any reduction in time use is effectively a constant drop in cost. You are literally shrinking either the width (time) or the height (electricity use) of the curve, as it were. For your point to be stronger, you would have to show that using lights for longer makes them less efficient, such that you increase the unit cost of electricity over the time variable. Which may indeed be the case. The naive model, though, is just Cost = (electrical use) * (time), and reducing time is the same as reducing electrical use via efficiency.