I often write glowingly about solar panels and how great it would be for me to control a giant laser beam. My plans for a solar powered empire work fine when the solar array is built in space, but odds are any significant solar arrays will be built first on Earth. So, after the sun goes down the power goes out! Our primary means of energy storage, what we need in order to light our nights, are chemical batteries. Batteries have gotten us pretty far, allowing us to carry mobile phones, laptops, flashlights, etc. But all these things work on a small scale. Sure, we have car batteries to jump start our engines, but in the end they aren't built to do much else on a reliable basis. Chemical batteries are volatile, toxic, and have a short lifespan. They wouldn't be able to power a solar civilization when the sun goes down, not until we see some chemical battery breakthroughs.
Till then we have the flywheel. Flywheels work like mechanical batteries, storing energy kinetically rather than chemically. They do this using a spinning wheel or disk. When energy is added to the flywheel, the wheel/disk is spun by a motor, when energy needs to be subtracted or utilized, the motor engages the spinning disk, generating electricity. A heavier disk adds a linear amount of potential energy storage. A disk capable of higher speeds, on the other hand, adds a squared amount of potential energy storage. Thus, disks meant to store a large amounts of energy must be able to spin at very high speeds (16,000 RPM, for instance). The trouble is, normal materials spinning at this speed explode due to the extreme forces acting on their surfaces. This is why modern flywheels are built using materials such as carbon fiber.
Several companies have started making large scale flywheels, but these days the most common use for a flywheel is power regulation. A large flywheel installation can smooth out bumps in the energy grid caused when power demand and supply fall out of sync. In the worst cases, a desynced power grid will cause a brownout or blackout, shutting down power completely. Residents of New York and more commonly, California, suffer from these disturbances. Unlike a generator, a flywheel can instantly respond to dips and jumps in an energy grid, preventing a frequency shift great enough to shut down the system.
Point is, while smoothing out our grids is a great idea anyway, I think flywheels could also be used to store and generate energy overnight. They aren't toxic, they recharge very quickly, they last for a long time, etc. Plus, they spin really really really really fast.