If we've built a Dyson sphere around the sun and have no more spare energy sources available in the solar system, then we can start thinking about reducing energy usage. If you cut back energy use by 50% for 1 million years and it delays this point by even 1 second, you've probably already lost out.

Until then, the energy your aunt Sally uses to run her AC pales in comparison to the amount of wasted energy being sent into space every second and so it's always worth doing research to capture that excess. There are plenty of rungs on the energy ladder before then, too:

https://en.wikipedia.org/wiki/Orders_of_magnitude_(energy)

In particular, capturing 100% of the solar energy hitting just the Earth would get you 10000x the US' electricity consumption in 2009. At those ratios, saving energy is just a bad idea compared to capturing more: If a key scientist has to spend even 1 day sorting his recycling or hunting for a more efficient AC, you may have already lost out.