For the whole Machine army to operate on only solar energy, the Machines should have huge solar panels. In our world, we reached 22 percent efficiency with Elon Musk's SolarCity, and while that's impressive, it's probably not enough to sustain the Machine army.
"But Cracked, it's sci-fi! It can bend some science in the name of story!" Of course! And that's the point: Let's suppose the pre-war Machines have access to 90 to 100 percent efficient solar panels. Who invented them? Fischer Morrow.
Fischer Morrow is an energy company in the Inception universe that's about to gain a monopoly over the world's energy supply. The CEO of a rival company named Saito knows that if this happens Fischer Morrow will become a new superpower. Maurice Fischer is a dying old man, and his son, Robert, will gain control over the company. Cobb (Chad Inception) and his team go into Robert Fischer's dream to implant the idea to end the monopoly and just chill the hell out for a bit.
Saito promises Cobb a get-out-of-jail free card and Park Place if successful.