The other day I was reading about the energy consumed and CO2 produced for each Google search. Energy efficiency is a hot topic and a lot of numbers (and hot air) are produced. Energy efficient data centers, green computing/green IT, household energy consumption, and much more is what is discussed in the news, IBM's theme is the Smarter Planet.
I am certainly interested in adding CO2 and energy metrics to database benchmarks like TPC-C, TPC-H, and also TPoX. But I am much more interested in the fundamental questions of daily life: Cash or credit (or in the US still check)? Not just only which is faster at the checkout, but what is greener? How much total energy is necessary to pay with cash and how much for paying with a card, including the post-processing?