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Solar: Infinite energy; Optimism; Growth and balance.
Punk: The status quo is failing; Something better is possible.
What Is Solarpunk?
Solarpunk borrows heavily from other *punks (cyberpunk, steampunk). It is a nebulous phenomenon. It can be described as a literary genre, an aesthetic, or even a movement. It has some recurring features:

Utopian Futurism
An emphasis on renewable energy, especially solar power (obviously)
A demand for technology and society to re-centre around sustainability, longevity, and balance, instead of the current demand for ceaseless expansion
The combination and juxtoposition of old-tech and hi-tech, of organic and mechanic
Decentralisation, community activism, and civic empowerment
Social justice
A recognition that economic, social, and ecological injustices are all deeply inter-connected
Solarpunks cherish nature and progress both. They do the same for the individual and the community. They blur imagination and reality. They believe in a world that is green, colourful, and bright.

For a more comprhensive description, check out this article.