Economic breakeven. Outputting power at a cost that is reasonably competitive with other sources of energy.
This is a VERY complicated issue. Beware of jumping to conclusions or mass-media simplifications. You cannot just look at the cost per megawatt-hour of electricity to compare technologies. Other issues include:
Availability. In Crozet, VA, the power company has a 1-MW Diesel generator in the middle of nowhere. I asked the power company about it. Based on the fuel costs and specs., anyone can see that costs more than other power company generators. However, it is used to meet peak demand a few hours at a time. It is quite cost effective for that purpose, because the equipment is so cheap.
Scale. The most expensive sources of electricity are pacemaker and hearing aid batteries. I suppose they must cost millions of dollars per megawatt hour, but people are happy to pay that much.
Expenses not accounted for; externalities. Smoke from coal fired power plants kills roughly 20,000 Americans per year, and makes life miserable for millions. I drove near a power plant in Georgia during an inversion a few years ago. It was like a thick fog, only it stank. I had to turn on the headlights. It enveloped towns, farms and schools for mile after mile. This smoke could easily be eliminated with scrubbers, but that would raise the cost of electricity by a few cents per kilowatt hour, and make coal uncompetitive.
Nothing will be done about this, because the people being killed and the children choking in the schools are poor, rural, conservative people who do not file lawsuits, and who seldom vote. Or they vote for the GOP, which advocates coal. They are voting to ruin their families' health and end their lives prematurely, with no benefit to themselves, because no coal is mined in Georgia.
If the airline industry were to crash airplanes killing 20,000 middle class people, or if MacDonald's were to kill 20,000 customers with food poisoning, these companies would be put out of business within days. But the power companies are careful to kill only poor people, and have been doing this since the 19th century, so they will not be held accountable.