May you explain this idea/issue a bit more?

Flywheels in real can save some power in some time.
I want to make Flywheels like in real, can save some power, and stabilize power.

That's a quite common suggestion from the Create discord.

I came to the Issues section moments ago to make the very same suggestion - it's nice to know it's being commonly suggested already. However, seeing how there's no proper description, I'd like to pitch in and provide some input on the matter (I didn't visit Create's Discord, so forgive me if some of this was already said somewhere before).
@Creepercdn, if you meant something else by your suggestion, please feel free to correct me.

As you might already know, flywheels in real life are used mainly as capacitors for rotational energy (in the modern world, with the help of electromagnets, they are used mainly to store electrical rather than pure mechanical force). I'm proposing for Flywheels in Create to perform the same function when they are not a part of a Furnace Engine.

I envision it like this: a Flywheel can store a certain amount of Stress Units per second, or SU/s - in the same way that IRL energy capacitors can store a certain amount of Watts per second (aka Joules). The more energy it has, the faster it spins. Conversely, in order for the attached shaft to act as an input adding energy to the flywheel, it must be rotating faster than the flywheel itself - and vise versa, if the shaft is slower, the flywheel will begin outputting energy instead, gradually slowing itself in the process.

Multiple flywheels can also be stacked together - the stack acts like one big flywheel with the total sum of their capacities.

While it can potentially store truly impressive amounts of energy, utilizing its full capacity isn't as simple as plugging in one Rotation Speed Controller at 256 RPM and calling it a day. As an input, the flywheel's Stress Impact is quite large, meaning that at large input speeds it will create a huge drain on the system, bringing all but the sturdiest generators to a halt.

Likewise, when acting as an output, it's Stress Capacity is directly proportional to its current speed: the faster it already spins, the more load it can support.

For small speed ranges, utilizing the flywheel is simple - just plug it in and let it smooth the load. But making the best use of its full capacity makes for a fascinating engineering challenge, requiring designing smart systems of switchable gearsboxes, step-ups and step-downs, smoothers and kinetic buffers.

This about sums up my vision of the idea. There are problems and details to be discussed, for sure, but I hope I've baked some good food for thoughts. :3