You don't "just hook it up to the grid". You have to work with the local utility company or companies to negotiate pricing, SLAs, and safety concerns (eg: shutting off power when linemen are working). Helion's fusion reactors are small enough to fit in a couple shipping containers. The idea is that the units can be mass produced in a factory, then transported to data centers and replace the grid hookup as the primary power source. Another potential market is stuff like remote mining operations which currently use diesel generators. The cost of transporting fuel to such sites can be quite high. A fusion reactor, while more expensive up-front, could be cheaper in the long run. And unlike solar cells, the reactor can be easily moved offsite when the operation is complete.