Yeah, I remember going to work with my dad (~20 years ago) at a datacenter for a large national bank. Power to the building was supplied by three large flywheel

Yeah, I remember going to work with my dad (~20 years ago) at a datacenter for a large national bank. Power to the building was supplied by three large flywheels, which were in turn (usually) kept spinning by electric motors driven by street power. If they lost outside power, they had three diesel engines that they could spin up and clutch in before the flywheels spun down too far.