I doubt that it is possible to answer this question accurately based upon the text alone (having read the statute in translation), and I doubt that there are even many lawyers or accountants in Germany who would know the answer if they don't work in this industry. The statutory language is ambiguous in these circumstances, and each of the possible readings could be reasonable in the fact pattern that you identify.
In practice, it would come down to what interpretation makes sense in light of the larger context of how the relevant utility operates its electrical grid.
If the grid were operated by the utility with meters that only disclose net power generation, then it is quite likely that this would not be treated as contributing to the grid.
But, if the grid were operated by the utility with meters that measured draws from the grid and contributions to the grid separately, it might be appropriate to treat the times when it did make contributions to the grid as contributions for purposes of Section 7.
A utility administrator or engineer would probably be more likely to know the answer even though, technically, it is a legal question.
I strongly suspect that German courts would be likely to defer to the utility company's interpretation of Section 7 so long as it had a clear, consistent and rational interpretation of what that term means.
On the other hand, if the utility treated one co-generation plant one way, and another one down the road differently, in the face of substantially similar facts, the court would probably decide which interpretation made the most sense under the circumstances itself, and would impose that interpretation on the utility.