This question is not on whether these devices work or not but so much as if they did work. What if electricity can be made so cheap that everyone could afford it ti the point there was no demand for electricity? Would a new clean source of electricity generated from home at a fraction of the cost of solar or other external power be harmful economically?
Energy firms generally have obsolescence in mind Meaning they're more likely to see trends in the energy sector and adapt. Whether completely shutting down operations across the board and possibly merge with a firm less invested in the obsolete industry, or discontinue unprofitable operations and alter investments toward improving this new cheap alternative energy. Both outcomes won't create quite the drastic economic shock, at least in the U.S. since U.S. workers in the energy sector in 2017 accounted for merely 2% of the U.S. population (https://www.energy.gov/sites/prod/files/2017/01/f34/2017%20US%20Energy%20and%20Jobs%20Report_0.pdf#page24).
Low-skill workers in this industry would be the ones hit hardest by the disruptiveness of this new technology. Unemployment may go up - bad for the economy. Yet the savings generated from this new economical method for producing energy may just overcompensate the decrease spending from workers and firms hit hardest by this new technology. Meaning a country's disposable income may rise, thus creating incentive to spend more.
Again, depending how disruptive this technology may be, I don't see any cost-saving method for producing energy at home or a business, as a harmful threat for an economy. There will be losers, sure, but the net-gain - economically, as a nation - in achieving energy independence outweighs the short-term losses from the descriptiveness of this new technology. Fusion reactors will soon be viable sources of energy and people in the energy sector ought to know this.