Know how solar panels take power from the sun? If they could build a fusion reactor it would be skipping the need for the solar panels, which aren't very efficient, and you now just have a miniature sun plugged directly into the power grid. The only fuel you need is Hydrogen which is BY FAAAAR the most abundant element in the universe (like 75% of all matter).

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You put a little bit of energy in to launch hydrogen atoms at each other and fuse them and the nuclear fusion reaction spits back out more energy than you needed to start it. Since we basically have infinity hydrogen (water is 2/3's H) that would mean we now have basically infinity energy. An extra bonus is the only "waste" product is helium which is pretty rare on Earth, actually used for a bunch of things (like cooling the particle accelerators that would be needed for the reactor), and is extremely safe inert gas that's easy to store.