An off-line uninterruptible power system and two line-interactive uninterruptible power systems are provided. The off-line uninterruptible power system and one of the line-interactive uninterruptible power systems additionally adopt a transformer for supplying an AC output, with lower voltage level than a rated output voltage, to at least one electrical device (especially those with resistive load characteristic). The other line-interactive uninterruptible power system controls its automatic voltage regulating circuit to supply an AC output, with lower voltage level than a rated output voltage, to at least one electrical device (especially those with resistive load characteristic).