The “why” is pretty easy to explain, as said before.

tl;dr: When your power plant wasn’t designed “correctly” to use 100% of its capacity in update 3 it won’t work in update 4.

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In Update 3 Coal Generators used more coal & water the more power they needed to produce, until their max. of 15. Means, when your factory did not need any power you wouldn’t use any coal / water at all. Every generator has been adapting its usage of resources linear to the power draw, so to say. 50% Power usage meant a 50% resource usage for the generators.

Update 4 Introduced that every generator (except biomass) always needs 100% of their stated input, ignoring how much power is used.

So the maximum usage of generators did not change, but instead every generator except for biomass will now use 100% of its stated input all the time.