I service the fire alarm systems for Microsoft here in the Seattle area; one site (a 200,000 sq ft server farm) uses a flywheel to bridge the time gap between utility loss and generator start. The flywheel is attached to an electric motor on one end that keeps the wheel rotating and generating power. When the utility drops out, the gen starts, syncronizes, and takes over on the other end. The result is 100% uptime, no fluctuations whatsoever. Pretty cool! The site draws about 12 Megawatts.

Greg