Many years ago I recall writing a special-purpose timer program for a Psion Organiser. I soon found that it ran the battery down rather quickly, and so began measuring the battery current. By inserting various strategically placed "sleep" codes into the program I was able to reduce the power consumption to less than 30% of the unoptimised code.

I imagine most optimisations for speed-efficiency (including use of lookup tables for frequent calculations) will also prove power-efficient. Unfortunately, for common desktop apps, coding for efficiency went out of the window more than a decade ago.