I don't want to beat you up about this but all I'm saying is that it appears you don't know why you got the message other than FreeNAS said your UPS was low.
The questions which I would want to answer if this happened to me are:
1. Did I have a power failure on the 1st of March and then how long did that power failure last?
2. What is the typical power draw on the system (determine how long a 390W UPS will last, meaning to a real test and time it)?
3. Do I have FreeNAS setup to power off within a few minutes or to run as long as possible before shutting down?
My concerns are:
1. Your UPS might be too small for your system setup. If you have monitors or other electronics then that pulls more power and you really do need to test it by pulling the UPS power cord out of the wall and time it. My 1000W UPS and my system with 4 hard drives doesn't last very long but long enough to handle brown-outs or short 20 minute outages.
2. You are replacing your UPS without knowing for certain it needs to be replaced, but if it's solely for peace of mind, then I fully understand that one.