I have a question regarding your math on the base-game accumulator and its energy density. You say that

In factorio, accumulators are based on Lead-Acid battery technology and have the energy density to match. Well done factorio devs!

But I'm arriving at a different conclusion. The base-game accumulator can store 5 MJ of energy and has a footprint of 4m². The height seems to be somewhere between 1-2m, making for a total volume of 4-8m³, disregarding parts that aren't actual battery. That makes for a best-case density of 5 MJ / 4m³, or 0.35 Wh/L. Meanwhile, Wikipedia quotes an energy density of 80-90 Wh/L for lead-acid batteries.

What am I missing?

https://en.wikipedia.org/wiki/Energy_density#Energy_density_in_energy_storage_and_in_fuel

I assumed 2x2x2 meters for the base accumulator. That page lists Lead-acid batteries as .56 MJ/L. So I did 8 * .56 = 4.64MJ, close to the 5 MJ in game! Nice!

...except, as you pointed out and I now realize, I never converted m3 to L. Oops! :'( That would mean an 8m3 lead-acid battery should hold around 4.5 GJ of energy. That's...a bit silly. And the flywheels I used are 5.3 MJ/L, so they are off by the same factor of 1000 that my lead-acids are. So...yeah. I don't think this mod needs to be 1000x more powerful than it already is. It's stupidly OP as-is.