a: To increase the reliability of the S.P.S., we suggest users choose a unit that has a rating of 30% more power than actual need. For example, if the system needs a 100W source, we suggest that users choose a S.P.S. with 130W of output power .or more. By doing this, you can effectively boost the reliability of the S.P.S. in your system. b : We also need to consider about ambient temperature of the S.P.S. and whether there is additional device for dissipating the heat. If the S.P.S. is working in a high temperature environment, we need to make some derating to the output power. The derating curve of "ambient temperature" versus "output power" can be found on our spec sheets. c : Choosing functions based on your application: 1. Protection function: Over Voltage Protection (OVP), Over Temperature Protection (OVP), Over Load Protection (OLP), and etc. 2. Application function: Signaling Function (Power Good, Power Fail), Remote Control, Remote Sensing, and etc. 3. Special function: Power Factor Correction (PFC), Uninterruptible Power Supply (UPS) function. d : Make sure that the model qualifies for the safety standards and EMC regulations you need.