Generally, smaller UPS units are not a good fit for that type of load. And, you want to make sure that you get a UPS that does a true sine wave output since the approximated sine wave output of this UPS would not work well with many non-computer loads. APC states: One potential issue is the inrush current draw for a motor. It can easily overload a Back-UPS UPS model. Secondly, these models of UPSs output a stepped approximated sine wave when on battery. This waveshape, while ideal for computer type equipment, is not particularly compatible with most motor loads. The result may be that motors run very slowly, erratically, or not at all when the UPS is on battery.
My general rule is that you want to get a UPS with the right type of output and generally at least 2X the running load of the motor to be safe. One sump pump that I just looked at was 120V with a 9.7A running draw, which is 1,164W, so I would say at least 2500VA so that it can handle the startup load. And a 3000VA UPS will only give you 15 minutes with a 1200W load on it. Extra battery units would give you around 30 minutes more each. You can see more on the approximate running times in this APC chart: https://www.apcguard.com/Runtime-Chart-for-Smart-UPS-XL.asp