I'm not an analyst, but until we hear from one, here are some observations:
a difference in scale
According to your source, the NATO attacks lasted 3 days, hit 5 power distribution stations, which "may take weeks to repair":
Three consecutive nights of air attacks caused extensive blackouts in Belgrade, Novi Sad, and Nis, the three largest cities in Serbia, Yugoslavia's dominant republic. In contrast with previous attacks on the power supply – in which allied warplanes triggered temporary outages by dropping carbon-fiber filaments that shorted out electrical lines – NATO forces this time struck at Serbia's five major power-transmission stations with high-explosive munitions, causing damage that could take weeks to repair.
The Russian Attacks in Ukraine were far more extensive. The Ukraine Energy Damage Assessment compiled by the United Nations Development Programme writes:
Ongoing war and targeted attacks on energy infrastructure have caused extensive damage across the country. Since 10 October 2022, Government of Ukraine estimates refer to more than 1,500 missiles and drones, as well as shelling and grenades, targeting the energy infrastructure of Ukraine, out of which more than 100 missiles are estimated to have hit large energy facilities. In the electricity sector, the generating capacity has been reduced by 61 percent, due to damages from Russian Federation missiles or drone attacks.
In 2022, the available capacity of Ukrainian power plants dropped from 36.0 GW to 13.9 GW. About 10 GW of installed capacity remains in the territories under temporary military control of Russian forces and is not delivered to the grid, including a 6 GW Zaporizhzhia nuclear power plant.
A total of 41 out of 94 crucial high-voltage transforming substations located in government-controlled territories have been damaged or destroyed by missiles or drones. More than half of these 41 substations have been hit more than once.
The destruction of the high-voltage grid makes it impossible to fully cover needs.
Continuous and regular waves of attacks on energy infrastructure continue to cause destruction and have already left 12 million people across Ukraine with no or limited electricity, disrupting internet communications as well as water supplies and heating systems at a time when temperatures had fallen below zero in most parts of the country. The average Ukrainian household had to endure five
cumulative weeks without electricity from 10 October 2022, to the end of December 2022, according to estimates based on Ukrenergo data.
That is, Russia hit both generation and distribution stations, all over Ukraine, during a period of 6 months. It destroyed 8 times as many power distribution centres as NATO did, and it destroyed them repeatedly. It also destroyed over half the power generation capacity, which NATO did not target, because (your source):
By focusing the attacks more on distribution lines than on main production components, the officer said, the damage should take weeks, not years to repair.
The overall damage was $10 billion.
Legal Assessment: Russia
From a legal perspective, the key question is whether such attacks meet the standard of proportionality, where damage to civilian objects must not be "clearly excessive in relation to the concrete and direct overall military advantage anticipated".
In an interview with the BBC, Michael Schmitt, professor emeritus at the US Naval War College, said:
An attacking force may hope that destroying the civilian power grid will lower the morale of the enemy, but that is not enough to justify the attack under international law. There must also be a concrete military advantage before the attack is deemed legal.
The sheer scope and scale of Russia's attacks on Ukrainian infrastructure make it unlikely that they can all be justified in that way, says Prof Schmitt.
"We're at a point now where they're hitting so many targets that I can't imagine they're picking power infrastructure that qualifies as a military objective in every case."
As a former US Air Force targeting officer, Prof Schmitt also doubts that Russia is fully validating every object it attacks - another requirement of IHL.
"You just can't conduct operations of that intensity and that frequency across an entire nation and have done your required verification of targets," he explains.
With that in mind, Prof Schmitt believes it is now "pretty clear" that Russia's main motivation, at least in some attacks, is to "terrorise the civilian population".
In the press release announcing the warrants against the Russian officiers, the ICC writes:
Pre-Trial Chamber II found that there are reasonable grounds to believe that the alleged strikes were directed against civilian objects, and for those installations that may have qualified as military objectives at the relevant time, the expected incidental civilian harm and damage would have been clearly excessive to the anticipated military advantage.
Pre-Trial Chamber II also considered that the alleged campaign of strikes qualifies as a course of conduct involving the multiple commission of acts against a civilian population, pursuant to a State policy, in the meaning of Article 7 of the Statute. As such, there are reasonable grounds to believe that the suspects also bear responsibility for the crime against humanity of ‘other inhumane acts […] intentionally causing great suffering, or serious injury to body or to mental or physical health’, as per article 7(1)(k) of the Rome Statute.
That the Pre Trial Chamber charged them not just with war crimes, but also crimes against humanity, indicates that they aren't just concerned that individual attacks may have been over the line, but see this as a possible systematic campaign targeting the civilian population. Put differently, the evidence for war crimes must be quite damning if they also qualify as crimes against humanity.
Legal Assessment: NATO
I have been unable to find a detailed legal analysis of the few destructive NATO power infrastructure bombings you mention. As far as I can determine, no formal charge of them being a war crime was made at the time. In particular, they are not mentioned in Final Report to the Prosecutor by the Committee Established to Review the NATO Bombing Campaign Against the Federal Republic of Yugoslavia.
The only mention of specific attacks on the power grid are the two transformers hit along with a TV station on April 23 (which the comitee looked into because there were 16 civilian deaths in the TV station):
Of the electrical power transformer stations targeted, one transformer station supplied power to the air defence co-ordination network while the other supplied power to the northern-sector operations centre. Both these facilities were key control elements in the FRY integrated air-defence system.
which were deemed legal targets.
Your question "but I'm not entirely sure if they applied different law standards" is a good one. I find that:
First, the definition of war crime has not changed; the relevant rules are from the Protocol Additional to the Geneva Conventions (1971).
Second, the Court had jurisdiction over all war crimes in that war, regardless of which nation committed them. And indeed, the Special Prosecutor did assemble a committee to review the allegations of NATO war crimes.
Ok, but did they look close enough? The committee describes its operation as follows:
To assist in the preparation of an Interim Report, a member of the Military Analysis Team reviewed the documents available in the OTP at the time [...]. The analyst prepared: a) a list of key incidents, b) a list of civilian residential targets, c) a list of civilian facility targets, d) a list of cultural property targets, e) a list of power facility targets, f) a list of targets the destruction of which might significantly affect the environment, and g) a list of communications targets. Very little information was available concerning the targets in lists (b) through (g).
They also write:
In conducting its review, the committee has focused primarily on incidents in which civilian deaths were alleged and/or confirmed. The committee reviewed certain key incidents in depth for its interim report. [...] The committee’s review of incidents in which it is alleged fewer than three civilians were killed has been hampered by a lack of reliable information.
And their conclusion writes
[...] NATO has admitted that mistakes did occur during the bombing campaign; errors of judgment may also have occurred. Selection of certain objectives for attack may be subject to legal debate. On the basis of the information reviewed, however, the committee is of the opinion that neither an in-depth investigation related to the bombing campaign as a whole nor investigations related to specific incidents are justified. In all cases, either the law is not sufficiently clear or investigations are unlikely to result in the acquisition of sufficient evidence to substantiate charges against high level accused or against lower accused for particularly heinous offences.
My interpretation is that the strikes on the power grid received relatively little attention at the time. Neither their number nor their method of execution looked suspicous to the investigators, and in the cases they did investigate, NATO provided an explanation identifying the power station as a legal target. The investigators could not definitely rule out that individual war crimes may have been committed, but they could rule out that severe or widespread war crimes were committed, and therefore saw insufficient public interest to conduct a deeper investigation, reasoning that even if they found something, it would not warrant prosecution in an international venue.
of international concern?
It's worth noting that a similar limitation exists in the ICC. Article 1 of the Rome Statute:
An International Criminal Court ("the Court") is hereby established. It shall be a permanent institution and shall have the power to exercise its jurisdiction over persons for the most serious crimes of international concern, as
referred to in this Statute, and shall be complementary to national criminal jurisdictions.
That is, the ICC is supposed to focus on the most severe cases, just as the ICTY was, and leave small fry to the national courts.
Conclusion
The much larger scale of the Russian bombing campaign makes it much harder to justify. It sure looks like Russians targeted everything they could, while NATO exercised a lot more restraint (for instance, in not targeting generating capability, and using filament bombs, which do little permanent harm, early in their campaign).
The much larger scale also attracted a much larger public interest, and made a high-profile conviction more likely, thereby meeting the threshold of "most severe crime of international concern" that warrants prosecution by the ICC.
Appendix: Why not Bucha?
Personally I was a bit surprised that the ICC indicted two Russian commanders for the long-range missile attacks in Ukraine, rather than e.g. for the more clear war crimes in Bucha etc.
I'm not. The war crimes in Bucha, clear and horrific as they were, were not widespread, and arguably not the result of a deliberate state policy. The perpetrators were soldiers and low level officers, but you couldn't secure a high profile conviction, and therefore couldn't meet the threshold "of international concern". Consequently, the prepetrators of Bucha remain in the jurisdiction of Ukraine - in fact, the first ones have already been tried there.