SPS's(standby Power Supplies) act as a surge suppressor or line filter , but they also contain a backup battery  and an inverter.   An inverter is a circuit device that makes AC power from the battery.  The inverter only kicks in when there’s a power outage  Small models have enough battery power for only a few minutes of operation, but that’s long enough to save your files and shut down unless your really brave and want to try and ride out the blackout.  Bigger models have multiple battery banks that can keep important equipment running for much longer.

    If you wish to spend a little more cash,  buy a true UPS. The inverter runs all the time, so there is no switch over time if the incoming power drops out or gets noisy.  Good UPS's have to have high quality inverters, and produces an output waveform as close as possible to an AC sine wave. Cheap "square wave" inverters are unsuitable for many applications and can, over time, damage  power supplies, and the "modified square wave" versions that have become popular recently aren’t a whole lot better, for continuous use.

     The bottom line, If you live out from larger cities, I would highly recommend a true UPS not SPS.  Be careful when you buy a UPS, as more then half of the stores I went to look at UPS's, wanted to sell me a SPS and said it was a UPS.  A UPS is exactly what it says it is, an Uninterruptible Power Supply.  If you live in a larger city, a good power surge strip will usually handle nasty surges and spikes.  Some even off er a warranty if it does not work and your equipment blows out.