I am guessing that you are looking at a UPS of some sort. Very few run with PURE Sine wave systems proportional thermostats usually will not work properly with modified or close to pure sine wave power systems (they will stick in 100 percent power and not shut down) The second part is that the ups devices are designed to give you enough time to save a file and shut down a system they often only run for a short time 10-15 min. not long enough to be useful. A back up genny is about the only way and at that a PURE sine wave one (honda is the only one that comes to mind.)
"What you want to look for is to make sure
when powered by the supply that the thermostat can shut off properly. This
is because the component in a proportional thermostat (triac) once fired in
the half ac cycle stays on until the voltage reaches the 0vac crossing point
of the sine wave. If the supply does not properly pull the sine wave to
zero then the triac would stay on into the next half cycle and you basically
have a runaway voltage scenario where its full on and the thermostat can do
nothing about it. This is why some cheap surge suppressors can cause
problems if the capacitors they use to prevent surge load the line enough to
not fully drop to zero volts."
Dion Brewington
Owner, Spyder Robotics
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