I live in the Florida Keys, and 15 years ago our power situation was only a little better than South Africa. Daily, hour long outages were common. It has improved a lot, though we're still at the end of a long extension cord.

I was part of a ISP startup that ran on a shoestring budget and the UPS being one of the places that we saved the $$$ to put into good modems. We used an 800kva UPS with dead batteries and two group 27 Lifeline sealed Lead Acid batteries ( series, for 24V ). We removed the dead batteries from the UPS, cut a small hole in the side of the case and added our own 4ga. battery cable extensions.

After using it for a bit, it seemed that the UPS's internal battery charger got confused by the additional capacity of the larger batteries ( though it's possible that it had an issue the whole time, and that's why the original batteries were dead ) and didn't fully recharge the batteries, so I added an external float charger to keep the batteries at the correct voltage.

It worked well, gave us something like 3+ hours of runtime ( enough for someone to run down and start the generator) and cost us about $425 ( $200 each for the batteries, $25 for the float charger- the UPS was free ).

After the ISP went under, it came to my house to run my entertainment center through hurricane Georges.