As a way to make use of resources you'd otherwise not have a use for this is interesting, but my feeling is that as a way to reduce "your" energy consumption it's misguided. I'm pretty sure that the energy usage of running even on very efficient dedicated hardware is going to be a lot higher than the incremental increase in consumption for an incredibly tiny VM instance in a data center focused on energy efficiency.

The hardware choice is arguably overkill as well, at least if wifi is available and on use. Run it on a repurposed old phone, eliminate its share of the power draw of a network switch, etc.