UPS’s are not created equal
I found that out personally when buying APC 1500 UPS’s that look like this — and found out the hard way that it would not keep the server up. So then we purchased the SMART-UPS, also rated 1500 and that model would keep the server up.
Mind you the normal 1500’s were good enough to keep the old server up, but I had to get the SMART-UPS to support the new server.
So keep in mind all UPS’s are not alike and spec accordingly.