Striking infrastructure sites, what would be considered as a war crime?

Question

During the Russo-Ukrainian war I saw that strikes on Ukrainian power stations were labeled as war crimes. Is that so? After all, electricity is a blood of industry, by disabling it you may cripple weapon production/repair. Same goes to fuel depots, cellular communication, roads and bridges to name a few. Clearly there are facilities that should not be damaged under any circumstances, like water supplies or medical installations.
So there are two questions. Is a power plant a legitimate target to strike? Is there a definition of what exactly could be considered as a legitimate target during the war?

Answer 1

Attacking a nuclear power plants without consideration of its consequences on the civilian population and/or the environment may qualify as a war crime.

Under the Rome Statute of the International Criminal Court (whose definition of war crimes reflects jus cogens, or preemptory norms of international law, even if not all nations are parties to the ICC), war crimes include

Article 8, paragraph 2 (b) (iv) Intentionally launching an attack in the knowledge that such attack will cause incidental loss of life or injury to civilians or damage to civilian objects or widespread, long-term and severe damage to the natural environment which would be clearly excessive in relation to the concrete and direct overall military advantage anticipated;

As to the definition of legitimate military objectives, the International Committee of the Red Cross has an overview on this subject. One succinct definition offered by the 1977 Protocol I to the 1949 Geneva Convention is

In so far as objects are concerned, military objectives are limited to those objects which by their nature, location, purpose or use make an effective contribution to military action and whose total or partial destruction, capture or neutralization, in the circumstances ruling at the time, offers a definite military advantage.

The same Protocol (article 54) prohibits attacks against objects indispensable to the survival of the civilian population (e.g. water supplies or agricultural land and livestocks for civilian purposes).

While electricity power plants do not necessarily qualify as indispensable to survival (they may be e.g. if required for adequate heating during winter) and can in any case be a legitimate objective, an attack on a nuclear power plant may violate Protocol I's requirement to limit the scope of destruction. Article 51 prohibits indiscriminate attacks

which employ a method or means of combat the effects of which cannot be limited as required by this Protocol; and consequently, in each such case, are of a nature to strike military objectives and civilians or civilian objects without distinction.