1.21 gigawatts.
Hill Valley.
Back to the Future Easter Eggs are two known Easter Eggs appearing in Grand Theft Auto V that contain references to the 1985 movie Back to the Future.
1.21 Gigawatts
This volt is the key to time travel discovered by Emmet "Doc" Brown. 1.21 gigawatts is the required voltage to power the "Flux Capacitor" which in turn powers the time machine. Considered impossible to achieve with the materials in the 1950s, Doc devises a plan to harness the energy of a lightning bolt to power the time machine. The easter egg is located on most power boxes throughout the game.
Hill Valley
Hill Valley is the name of the fictional town in the movie Back to the Future. The name itself is an oxymoron, which was employed along with gags in the movie. The sign is located in Pacific Buffs, Los Santos near the Vinewood Cemetery.