This report examines the relative merits of Diesel Rotary UPS (DRUPS) versus the more traditional Static UPS combined with standby generators. The efficiency of static UPS systems has improved greatly over recent years and now is on a par with the efficiency of a DRUPS system, so one of the main drivers for adopting a DRUPS solution has been removed. DRUPS systems still tend to be more space efficient than Statics, however the capital costs remain higher. Lifetime costs over a 10yr period are approximately equal, depending in large part on off-peak efficiencies of both systems. Over a longer period DRUPS would still have the advantage, mainly because of the costs on replacement batteries for Static systems.