As another idea to help with things, I wonder if it would be possible to toss some solar panels on the roof, as well as go with some larger 6 volt solar batteries.
The size of both can vary, but I'd probably get enough panels to completely charge the batteries in one day (around eight hours), then have enough battery amp-hours to run the sump pump for at least 4-7 days. That way, the panels will always be ahead of the pump's energy use, even in bad weather. To boot, add an AC charger so if there is mains power, the batteries will always be topped off.
What this will provide is a "guaranteed" 4-7 days of sump pump usage if power goes out, then if the solar cells are done right, the battery powered sump pump could run indefinitely off the panels.
If snow is an issue, the panels can be faced at 35 degrees or steeper which would help it fall off, and there are places which sell heat tape to help with keeping the panels clear.
