The company said the 14.4MW project is planned to go into operation at the end of the summer.
The 26,500 solar modules will be built into 1.4-kilometre-long strips.
The in-site lignite mining is expected to stop in seven years leaving behind an extensive lake.
Katja Wünschel, Chief Executive Officer Onshore Wind and Solar Europe and Australia at RWE Renewables, said: “We are investing €11 million (£9.2m) in our new project at the Inden opencast mine.
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