The short answer, I think, is no. Even a small sump pump pulls something like 6 amps (720 watts) so you'd need a pretty large UPS to keep on running for any period of time. From APC's runtime chart a 1500 VA back-up (retail $250) would keep 700 amps running for 5 minutes. Motors have a fairly large in-rush of current on start-up too. I don't know if a UPS is designed to handle that. The pumps sold with battery back-up systems are 12 volts DC, so you couldn't just plug one into a UPS either. It is probably far more cost effective just to buy a automated battery backup sump pump system.

You shouldn't be using a trickle charger. You should be using a "battery maintainer" that senses when the battery actually needs to be charged, does the charging and then shuts off 100% once the battery is at full charge. Your batteries will hopefully have some sort of removable caps. If so, go buy a five dollar hydrometer and a thirty dollar battery load tester. These two devices are essential tools for monitoring the ability of your batteries to do their job when called on. The hydrometer will allow you to extract the same amount of battery fluid from each cell and measure the specific gravity of that mix. That measurement will tell you the state of charge in each cell. A fully charged battery will have almost the same specific gravity at the top end of the scale. If all the cells but one read as being fully charged, then that tells you that the cell is bad and the battery needs to be replaced. If all the cells show a low state of charge despite being on the charger continuously, then the battery is just worn out and must be replaced. The load tester does exactly what it says. Batteries are rated in CCA or Cold Cranking Amps. This test shows the batteries ability to put out a high amount of amps for a specified period of time. Think of it as "reserve capacity". You could put a voltmeter across a battery and it might read 12 volts; leading you to believe that it's OK. But a voltmeter won't tell you how long a battery will deliver 12 volts to a motor that draws ten amps. Amperage is the load and the load tester allows you to test for that. In other words, you could have a battery in your flashlight that will make the bulb shine brightly when it is first turned on but if that light goes out while you are trekking to the outhouse at 3.00 AM, then it has no capacity left. Of course, this is exactly what you are wanting to avoid and those two testing devices PLUS taking the time to learn how to use them properly, will put you in control of your batteries, instead of the other way around.

This is similar to a solar system with battery backup minus the solar.There are others, but I think Xantrex makes a unit like this, also Outback. Basically they are invertor/chargers that use the AC to charge the batteries as well as pass thru to a 'secured' load. When the AC fails the unit uses the batteries thru its inverter to run the load. Usually these are not rated as a UPS because the cutover time isn't quite fast enough. With the addition of solar, the cells charge the batteries rather than the line AC.

Sorry for a late follow-up here (been traveling) but I found a picture, without too much more information, of the transfer switch which you have. It appears it has wired cords on the two "hard-wired" connections and is basically designed for RV/marine applications where this type of connection is common. I'd simply put a plug and outlet connector on the appropriate cords and make this a plug-in device. Not sure about the 300 watts being able to power the pellet stove (fans, igniter, and auger) as pointed out above, and an alternative power-source to recharge the battery during an outage. Just had our first storm last night (early in the season) and while I do have a pretty good portable generator/hard-wired transfer switch setup, I wasn't quite ready for 11 hours of it... however, the gas cans are now full!