Re: Small business support
The problem with the "one PSU in the UPS, the other direct to mains" is that most people don't understand the details and what it means in terms of load capability and runtime.
During normal operations, only half the power goes through the UPS, so the UPS will test the battery, do some runtime calculations, and they will be out - not just by a factor of two, but maybe by a factor of 5. This also means that any "run until the battery reaches x minutes" shutdown config settings are at best questionable.
Bu the real biggie is when people look at the lights, see that the UPS is only (say) 40% loaded and add something to it. Everything still works fine - until the power goes off, the UPS goes into overload and turns off.
I have actually witnessed someone who I previously thought knew better, when adding a server to what could only euphemistically be called a rack, put one mains lead into a mains feed, and try the other lead in each of the UPSs in turn till he found one that would take the extra load without beeping. Yup, all the UPSs were at or very close to 100% when only supporting 50% of a significant number of the servers and were guaranteed to just go "clunk" if the mains went off. Mind you, they were all Armenian Potato Clocks (I'd never heard them called that before, love it !) with dead batteries ...