For some, it's also political -- a way to leave their mark on a city they feel has turned its back on them.

CNN: Wall art masquerades as class war

"As much as it's an effective, common-sense source of some sustainable electricity, it's also a great way for people to engage with the issue of sustainability ... to feel like they are part of the solution in a very immediate, fun and visual way that doesn't make you do anything you wouldn't already be doing, " said Miller.

CNN: Green sidewalk makes electricity -- one footstep at a time