Ask Tom Holt, president of search engine company Surf Wax, to explain uninterruptible power supplies (UPS), and you get an education.\u201cUptime is critical for us,\u201d says Holt, who has a data center in Menlo Park, Calif. When Holt put in a generator, he soon realized he would need another type of UPS. Because a generator will often take a few minutes to start up when power is lost, a UPS needs to supply electrical power in the interim. Holt had concerns with the generator.\u201cThe voltage you get off a generator varies greatly,\u201d says Holt. \u201c[The previous] UPS had no voltage regulation whatever, so we would get spikes off our generator, which causes havoc with our UPS units and the systems attached to them. We needed to find a UPS that gave us good filtering and kept the voltage in the proper range.\u201dHe chose gear from Falcon Electric, which manufactures an online, sine-wave UPS.The online UPS doesn't just pass the electrical power through; it regenerates a whole new utility source. Online UPSes cost a little more than interactive UPSes, but are necessary in data centers, Holt says.\u201cThere's an old ad from FRAM oil filters that says 'You can pay me now or you can pay me later.' UPSes fall into that category,\u201d he says. \u201cThe price is really nominal. With Falcon you are getting the high end.\u201dThe Falcon Electric SG Series 1.5 kVA UPS Holt chose provides back-up power to the servers in his data center before the generator takes over. Typically generators don't kick in for four to 24 milliseconds.Holt had been using a line-interactive UPS.\u201cOur local power company, PG&E, has brownouts, and there is a lot of construction in the area, so we were getting shutting down periodically,\u201d Holt says. \u201cThat's why we got the generator. If you are going to use a generator, the Falcon unit is exceptional.\u201dThe online, sine-wave UPS can handle frequency and voltage shifts, whereas some line-interactive units can't. It is good for all types of power problems, including power surges, voltage spikes, brownouts, blackouts and line noise.Online UPSes are also made by Liebert, American Power Conversion, PowerWare and MGE.