|First of all always look for well known suppliers, that will give you better guarantee.
Then look for a true sinusoidal UPS, which means that the output is sinus shaped resembling the line waveform. The cheaper square-wave devices can create problems to switching power supplies of PC and other electronic devices. For more or less the same reason check the minimum allowed load power factor, this parameters directly deals with current phasing of inductive loads, but for electronic devices lower power factors means that the UPS can tolerate higher waveform distortion and the harmonics generated, again, by switching power supplies of electronic devices.
If you are looking for medium-high power UPS's looking at characteristics you'll find also an harmonic THD distortion, the lower the best.
About surge protection don't worry too much, an UPS device by default give surge protection: A surge is a fast transient overvoltage of power line (up to 200-250% of nominal value), because an UPS output is controlled and regulated you should be theoretically free from them.
Another important characteristic is the autonomy: how many minutes of power it can deliver at nominal power. This defines the dimensions of the battery pack. Choose the right time you need, the batteries are very expensive and are also delicate. Remember that batteries have a lifespan (that depends very much on ambient temperature, higher the temperature shorter the life), good quality batteries will guarantee a life of 18-24 months at 25°C +/-2°C.
modified 14-Jun-15 15:55pm.