Doc: "Marty, I'm sorry, but the only power source capable of generating 1.21 gigawatts of electricity is a bolt of lightneng"
Marty: "What did you say?"
Doc: "A bolt of lightning. Unfortunately you never know when or where it's ever gonna strike."
Marty: "We do now."
Doc: "This is it! This is the answer. It says here that a bolt of lightning is gonna strike the clock tower at precisely 10:04 p.m. next Saturday night! If we could somehow harness this lightning, channel it into the flux capacitor, it just might work. Next Saturday night, we're sending you back to the future!"