By Reuters Staff 2 Min Read FILE PHOTO: Solar cell panels are seen on the hood of German solar car company Sono Motors' prototype car "Sion" in Munich, Germany, August 1, 2018. REUTERS/Andreas Gebert FRANKFURT (Reuters) - German solar car firm Sono Motors is exploring a U.S. stock market listing that may value the company at more than $1 billion, people close to the matter said. Sono Motors is developing an electric car that uses solar panels on its outside to power its batteries, but which can also be charged using a plug. The solar panels generate energy worth on average 112 kilometers a week.