Solar-powered, as in, with a solar battery? Or solar powered, as in, "respecting celestial mechanics" (like a sundial).
You decide. But we know one thing for sure: WWF's latest billboard warning about ocean levels rising brings the point home dramatically.
Some assumed that the board is powered by electricity and hence contradicting its message, but it's not "powered." Instead, the movement of the Sun over the course of the days casts an interesting shadow over the board. See the time-lapsed video below.
One YouTube user, LowerCO2, sheds more light on how the board works:
"The board respects celestial mechanics (Kepler Rocks!). It's perpendicular to the equator, with an unobstructed exposure to the west. The "waves" start at about 12:00. The challenge was not azimuth (the daily path of the sun), but altitude, which due to the Earth's tilt, required the scalloped awning's shape to be distorted to compensate for 43N latitude, during the life of the posting (about 8 weeks). Thanks to CBS Outdoor, and PLEASE support the WWF."