Nice article about how EBay created a way to measure the cost (power, etc etc) or each transaction. And then found ways to improve the software to reduce those costs, saving 2 million. This seems to be a dying art (designing software for efficiency) these days, and I only see it in embedded systems the most where you need to squeeze the most out of hardware. Note this does not include smartphones.
http://www.forbes.com/sites/michaelkanellos/2013/03/21/ebays-mpg-for-data-centers/