A Standby UPS system is the most common type of uninterruptible power supply system in the market today, and is often referred to as an offline UPS system. It is cheaper compared to the other types of UPS systems and is normally used for personal computers.
It is called a standby UPS system since the inverter and the battery do not supply any power unless the main source of power goes out. It essentially remains on a standby mode unless it is needed. The main source of power used by this system comes from a utility or power line. The system has a transfer switch that automatically selects the backup power provided by the battery once the main source of power goes out.
When the main power source goes back on again, the same transfer switch would automatically select the utility or power line as its source of power. The battery of the system is charged using the power from the utility or power line when the main source of power is being used.
One issue raised against the use of a standby UPS power system is the time it takes to switch from one power source to another. There may be some instances where the switch is not fast enough to ensure uninterrupted use of a PC system, but this situation rarely happens.
Aside from being cheaper, a standby UPS system is also efficient and does not occupy much space inside a home or office. This system is mainly designed for general consumers or ordinary users and is not normally used for devices with a very critical function, like a server.
References:
http://www.pcguide.com/ref/power/ext/ups/typesStandby-c.html
http://www.apc.com/us/en/faqs/FA157448/
http://www.gocsc.com/uploads/white_papers/CDA24729556646388C183D4320810EF7.pdf