As the ICC statement makes clear, the bombing of power plants and other energy infrastructure in and of itself is not a crime. It only reaches that level if the strikes are committed with the intent to cause civilian harm and suffering and are not justified by a military advantage.

The NATO bombing of power infrastructure in Serbia was investigated by the United Nations International Criminal Tribunal for the former Yugoslavia (ICTY). The ICTY prosecutors were satisfied with the evidence presented by NATO that the targeting was narrowly aimed only at infrastructure which specifically fed into key parts of the military air defence and command and control network, and that it had hit transformers and other easily replaceable elements rather than the main plants. [0]

So it was proportional, fulfilled a military purpose, and wasn't aimed at causing undue suffering of the civilian population.

[0] https://www.icty.org/en/press/final-report-prosecutor-commit...