| My father is afraid of his treadmill suddenly stopping while he's | walking due to a power outage. I figured that a ups backup could give | him some time to step off before it stopped. But I figured that the | least expensive one with the lowest power rating would work, since I | assumed that the power rating was only to give you an estimate of how | long the unit will power an item. | | Since most of the ups units offer several minutes for a low load, I | figured that it would give a more than adequate 10 to 20 seconds at | the higher load of a treadmill. | | Will it work or will something trip inside the unit if the load is | greater than the rating?
DC to AC power inversion tends to have very little overload headroom above the nominal maximum operating range. Generators have more, but not a lot more. Your utility mains electric supply has a lot more overload capacity. They have some "big honk'n generators".
Even if the UPS does not trip when running on working mains supply, it may well just quit when overloaded in DC operating mode when the mains goes out.
The only (IMHO) safe way to run a motor on a UPS is to get a UPS rated for operation at the motor's LRA (lock rotor amps). And that would be a rather large UPS.
FYI, I have found that running the circuitry of a house on a UPS, even a large one rated for a whole circuit capacity, could well be unsafe because it is unable to deliver the current (but at some rating levels would not trip itself off, either) to magnetically trip building circuit breakers in the event of a short circuit. Even a small generator can be unsafe for just such a reason, or even a medium generator for a whole house (does it have the ability to deliver the current needed to trip the main breaker in the event of a fault that needs to trip the main?).