Smart Power System‘s standby UPS solves power outages and typically offers surge protection. This UPS design simply passes utility power until either a power failure, voltage sag, or over-voltage condition occurs. The standby UPS will switch to battery mode when these events occur until utility power returns to a normal level. This standby is the most cost effective and typically makes use of a square wave or modified square wave when operating in the battery mode. These units are typically found in units 600 VA and below and are designed for home use or a single computer/terminal in a business application. The disadvantage of a standby unit is that voltage sags, brownouts, or over voltage conditions will cause a standby UPS to switch to battery mode, and may cause it to completely drain the battery and shut off even though line voltage is still present.