They supply power through a back up battery and a DC/AC inverter or DC/DC converter. A UPS is inserted between the source of power and the load it is protecting, so that when there is a power failure or problem, the UPS switches from utility power to its own power source almost instantly.
Uninterruptible Power Supplies – Triad Magnetics
Three main types of uninterrupted power supplies are currently used. Offline UPS systems are standby systems that provide battery power to equipment when the main power supply falls underneath a certain limit. These are mostly used in home and office applications due to their lower cost. A line interactive UPS, on the other hand, switches to battery mode during a power outage, like the offline UPS. The difference is that the line interactive UPS actually boosts the main power supply when it falls. This type of UPS is mostly used in corporate applications.
Finally, on-line UPS systems, which offer the highest level of protection to electrical devices, are often used with medical technology or other sensitive equipment. These systems convert AC to DC and then back to AC in order to supply critical power loads. They have automatic bypass so that the power is continuous during an overload or UPS failure.
Uninterruptible power supplies are used to protect computers, telecommunication equipment, and any other electrical equipment that could cause injuries, fatalities, or serious business disruption and data loss if unexpected power disruption occurs.
UPS systems can be either a stand-alone unit, which is external to the equipment being powered, or a battery back up, which is implanted into the equipment. They can also come in different sizes to back up a single computer or an entire data center or building.