Most of UPSs up to 2kVA are designed to work without an additional fan. For bigger models cooling is required to ensure stable operation. Each UPS will have product-specific automatic logic to control the fan. Line Interactive UPSs will usually switch off the fan during normal operation in line mode, but online UPSs due to double convertion need to run it permanently. You may notice reduced speed if conditions allow, but the fan cannot be switched off at any point. More details about the fan are mentioned in the datasheet
