Online UPSs offer "ECO mode", which significantly reduces own power consumption. Naturally it comes at a cost. When running in ECO mode, the UPS will in fact run in bypass mode with adjustable limits. This means as long as the input voltage and frequency are within set limits (i.e. 49-51Hz and 220-250V), it will pass through the input voltage directly to output. Once the voltage or frequency goes off the limit, it will turn on the inverter and switch to double-conversion mode. Additional drawback to consider in this setup is the transfer time (usually few miliseconds) between ECO mode and LINE mode (double conversion).
