They cite the Solar Foundation, a nonprofit, which counted 142,698 employees in November of 2013 who spend "at least 50% of their time supporting solar-related activities." The foundation says they made 74,000 phone calls and sent 11,000 emails to produce their workforce census.
For coal, they reference a 2013 report from U.S. Mine Safety and Health Administration saying there were 123,227 coal mining jobs in the U.S.
For now, the workforce for the coal industry writ-large — those who work in transportation and maintenance, for instance — remains larger than for solar, Politifact says.