The different types of UPS systems
The Standby type is the most common and economical type of battery powered UPS system available. With this type of UPS the powered equipment is connected directly to the utility power line without any form of voltage regulation. At the same time the UPS is charging / maintaining its batteries via an AC to DC converter. When the utility power fails, the DC to AC inverter of the UPS will turn on and supply the load with power in a matter of milliseconds.
Most people believe that since the equipment is connected directly to the utility power line it can be easily damaged by a power surge or similar phenomena and the UPS works as nothing more than a short-lived battery so that you can save your files before turning off your computer. That may not be entirely untrue, because a high quality Standby UPS always has a good surge suppression circuit, fuse and also provides more than adequate noise filtration but very low cost / quality products rarely ever feature even the simplest forms of protection. The very high efficiency, simplicity, small size and low cost of the design make it ideal for home and small business users.