Query: When did Tucson's main power plant begin operation?
Context: Tucson's primary electrical power source is a coal and natural gas power-plant managed by Tucson Electric Power that is situated within the city limits on the south-western boundary of Davis-Monthan Air-force base adjacent to Interstate-10. The air pollution generated has raised some concerns as the Sundt operating station has been online since 1962 as is exempt from many pollution standards and controls due to its age. Solar has been gaining ground in Tucson with its ideal over 300 days of sunshine climate.