The problem with UPS systems is not being able to get one with enough power to run a pump, but that most only produce a "modified sine wave" a square wave that motors don't like and run at about 60% of there speed.
A slight digression here.
In the early days of inverters, there were some that did have a square-wave or quasi square-wave output. Certainly, they did increase motor losses over pure sine wave operation. For a particular motor, you had to derate it some in terms of rated output power because of the increased RMS current. How much derating depended on the inverter topology and the motor pu reactance.
The advent of IGBTs from second generation onwards means that now, a reasonably good sinewave output can be synthesised using PWM techniques. With a little filtering, which many have built in, you often get a better wavevorm, better voltage stability and closer frequency tolerance than that obtained from the utility.
As I mentioned, it was a digression but in reference to your comment about the output waveform..
The main limitation with running a motor from a UPS is not quality of output. It is the capacity to deliver the required motor starting current.