Quote Originally Posted by Ranga View Post
I've done a bit of research, and it appears there can be issues in connecting batteries to a UPS that are much larger than what the UPS is designed for.

This can cause overheating and inability to charge the batteries.

I think I'll end up sticking with the charger > battery > inverter > pump solution.
You are probably on the money then, just remember good inverters have a "sleep" mode so they draw very little current when idling. I would use a "Selectronic" inverter or similar as they are very robust.