MIT figured out fusion this month, water shortages are on the verge of being solved since water is not scarce, just dirty/salty - it's an energy problem.
Let's pretend a fusion plant is completely free to build, maintain, operate, and decommission.
50% of the cost of electricity is from transmission and distribution. Those costs aren't going anywhere.
So at most, you'll get a 50% savings. But you're not getting that either, because that fusion plant definitely wont be free.
Fusion reactors are mechanically simple and safe and can be made in tiny form factors, including engines. They are predicting container-sized "plants" that can power hundreds of thousands of homes. Fusion will heavily decentralize the grid (partially because of transmission costs).
Nothing is totally free of course, but fusion can do for energy what the internet did for distribution, which we can round to having made it free.
ianaEE
The cost per watt of fusion will come from the expenses associated with making fusion plants, averaged over their lifetime; additionally you need something to fuel the fusion reaction.
I doubt that fusion is going to be cheaper than solar anytime soon, given that solar has the advantage of an established manufacturing process, while fusion is still in the research phase.