TOKYO—Panasonic Corp. said it has begun to sell some of the output from a solar-panel factory it operates with Tesla Inc. to non-Tesla customers, another sign of the uncertain outlook for Tesla’s SolarCity subsidiary.

Panasonic said it began making solar cells and modules at the factory in Buffalo, N.Y., in August 2017. The companies have said the factory’s cells and modules would be used in Tesla solar products. Panasonic said it was now selling some of the plant’s solar equipment to other customers, while Tesla is still...