Hi Ludwig. A dataset that lacks a license is under all-rights-reserved copyright by default. No ifs or buts. This has nothing to do with Germany, it is mandated under the 1887 Berne Convention. Specific details related to OPSD use cases are covered in the legal opinion Till Jaeger wrote for the project in 2017. The PSI re-use submission I recently coordinated for the European Commission (Morrison et al 2017) deals with the downsides of unlicensed public data in quite some detail. On the upside, ENTSO-E is aware of our concerns and is attempting to address them. If you want open electricity data to stock the OEP database, try the French TSO Open Data RTE site which employs a CC BY 2.0 compatible license.
Note too that Elsevier, for instance, requires a statement from the lead author that the authors hold all necessary rights to the uploaded material, including any supplementary data. Some authors, I hear, happily tick that box when this is not the case. I doubt their legal departments would allow this if they were aware of the practice. HTH, Robbie.
Jaeger, Till (24 July 2017). Legal aspects of European electricity data — Legal opinion. Berlin, Germany: JBB Rechtsanwälte.
Morrison, Robbie, Tom Brown, and Matteo De Felice (10 December 2017). Submission on the re-use of public sector information: with an emphasis on energy system datasets — Release 09. Berlin, Germany. Published under a Creative Commons CC BY 4.0 license.