An interface between two systems at which (a) they are not connected physically and (b) any logical connection is not automated (i.e., data is transferred through the interface only manually, under human control).
The response you received from our tech support team relates to the power on self-test that our UPS performs when utility power is restored to a UPS following a power outage that lasts long enough for the batteries to be completely drained and the UPS powers off. When the UPS begins the power up sequence it performs several self-tests.
This includes a check of the battery strength. As with all batteries a UPS battery will become weaker as it ages. This means that a battery that has been 100% drained may not have to pass the self-test immediately after the power is restored.
Failure of this test prevents the UPS from starting up, because the batteries do not yet have sufficient charge to support your equipment if the power were to fail again immediately after the restart. This could result in your equipment crashing.
Following an outage, the UPS restarts automatically when AC input power returns (unless the restart function has been disabled) and the load is supplied again.
I have tested with up to a 1,500 watts constant load, and it was able to power a 1.5kw load for around an hour and a half.
It should be good for 20 years or so. Expensive, but, damn, peace of mind is nice. Under typical load, it can power my entire rack from around 4-6 hours depending on load....