Deciding what you want the system to do for you will determine what solution is best for you. 

• A Voltage Stabiliser (including an AVR), will increase the mains voltage when it goes low, & reduce it when it goes high. It does not offer any backup when the mains fails.
• A Voltage Switcher will disconnect the mains voltage from your equipment when the mains voltage goes outside set limits.
• A UPS or Inverter System will give you uninterrupted power for a limited time when the mains power fails, or when the mains voltage goes outside set limits.

To get instantaneous power backup, one can purchase either a UPS or an Inverter system. Traditionally, a UPS was meant for short backup time for computer equipment, whereas an Inverter system was designed for long backup time for home use. A UPS will usually come as a complete unit with batteries, etc, whereas the components of an Inverter System are usually supplied separately, & then assembled at the site. This includes the inverter charger, the batteries, the battery cabinet, etc.

For an office environment, a UPS may be more suitable, especially due to its neat appearance, plus it offers several additional features, e.g. Power Monitoring & Shutdown Software, etc.

For home use (where one wants to power items such as lights, TV, music system, computer, etc), an Inverter system is usually more appropriate, especially due to its simple design & longer backup time (which can easily be increased).

Some equipment require a certain quality of supply voltage waveform in order to operate properly. The best quality voltage waveform is a pure sine wave (as opposed to modified sine wave). A UPS or Inverter that produces a pure sine wave voltage waveform is more expensive than the modified sine wave version. It is critical that you know what waveform your equipment requires in order to worlk properly.

There are other factors that can determine the type of backup system to opt for, but the above are the key ones.

The size of system you purchase will determine the minimum & ideal size (power rating) of the Inverter or UPS.

If you purchase a system that is too small, it may switch off when there is a mains power interruption, due to it being overloaded. It may also get damaged or at the very least will not last as long as it should, due to being overloaded.

If you purchase a system that is too big, you will have spent an un-necessary amount of additional money. The physical size of the sysem may also be a constraint. In general, the bigger the rating of the system, the bigger the space it will require, & the bigger the cooling system, etc.

The ideal size of the system should be one that allows you to comfortably power your existing equipment, plus allow limited expansion, e.g. 30%. You thus need to factor in any likely expansion you may have during the next 3-5 years or so.

For a fixed load, the total backup time is determined by the capacity of the batteries installed. The type of batteries (& their condition) will also have an effect on the backup time. Different batteries may have similar amp-hour ratings, but may differ in their performance. One key factor is the rated depth of discharge of the battery (ie how deep can the battery safely discharge without getting damaged). This can vary from about 20% to about 80% for the best batteries.