That *will* likely run a sump pump. Moreso than the inverter. It's godawful inefficient, but you might be best off trying to charge the UPS off the inverter (with no draw on the output side), ...

Plus, advice on whether to pay more for an inverter model, or even invest in a whole-home generator. ... If your sump pump stops and your basement floods, you’re looking at a costly repair.

1500W Sump Pump Battery Backup System, Pure Sine Wave Power Inverter with Charger, Backup System for Power Outage, LCD Screen Remote Controller, Automatic Switching for Grid and Battery ...