The DoD LoW (law of war) Manual (¶ 5.6.8.5) says:

“Electric power stations are generally recognized to be of sufficient importance to a State’s capacity to meet its wartime needs of communication, transport, and industry so as usually to qualify as military objectives during armed conflicts.”

Russia has committed a plethora of war crimes and I've been all for giving Ukraine everything they ask for. I'm also sure the Ukrainian government has backups for electricity but I've never been convinced that attack the electric grid is itself a war crime unless we could somehow prove Russia is doing so with the express purpose of harming civilians. (Which let's be honest could 100% be the reason)