First Solar, a leading maker of solar panels, based in Tempe, AZ, has announced that it will build an enormous, 2,000 megawatt solar power plant in China, starting next year. Bloomberg reports that it will be the largest solar power plant in the world.

The move is evidence of the falling prices for photovoltaics, which convert sunlight directly into electricity and have historically been seen as too expensive for very large power plants. But although the new power plant will be large by historical standards for solar, it’s still a tiny fraction of the world’s total energy needs, which number in the millions of megawatts. And some experts warn that the kind of solar cell that First Solar makes cannot be made in the large numbers needed to supply a large part of the world’s electricity demands.

Typically, utilities have favored solar-thermal technology, which use heat from the sun to make steam that drives generators, for large power plants. For example, in California 6,000 megawatts worth of large solar projects are under review by the federal Bureau of Land Management, and all involve solar thermal. But First Solar, which makes “thin-film” solar cells, has been able to sharply drive down the cost of making photovoltaics, and is known for having the lowest manufacturing costs in the solar industry. (This is difficult to confirm, since not all solar companies publicly disclose their costs.)

However, First Solar’s cells use tellurium, a relatively rare element, which could limit the number that can be built in a year. And, according to one recent report, the availability of tellurium could limit worldwide production of such cells to about 10,000 megawatts per year, even though very little tellurium is used in each solar cell.

First Solar has disputed concerns about the availability of tellurium, saying there is plenty to be had for the foreseeable future. But to reach millions of megawatts of solar cells, we will likely need a variety of solar cells, especially ones based on common materials such as silicon.