Frosty... it's a 3000VA UPS, would you care to explain how it could be using transformer(s) rated at less than 3000VA, seeing as how all the current goes through them? It begin an inverter it is by definition a switching power supply, just as in compact fluorescent ballasts are smaller than line frequency ballasts because of the increase in frequency the transformer size is much smaller. Filtering from the natural inductance of the coil, and likely some other added filtering make the output appear as 50/60hz.