Solar energy:

Beginning with the surge in coal use which accompanied the Industrial Revolution, energy consumption has steadily transitioned from wood and biomass to fossil fuels. The early development of solar technologies starting in the 1860s was driven by an expectation that coal would soon become scarce. However, development of solar technologies stagnated in the early 20th century in the face of the increasing availability, economy, and utility of coal and petroleum.

Please answer a question about this article. If the question is unanswerable, say "unanswerable". What slowed the development of solar technologies in the early 20th century?
increasing availability, economy, and utility of coal and petroleum