Look for the APC Smart UPS from the XL series if you can get them.
They are the same, but have an extra connector on the back for
external batteries, and you can tell the UPS how many batteries you
have connected. I believe they are 24v for 1500VA and under and 48v
for the 2200VA and larger.

I have a SmartUPS 750XL that I bought of eBay for the grand sum of £2
(yep two quid!) because it had no batteries installed. I didn't care
because I was going to add a large external bank anyway. I have 4 CSB
battery 12v 75ah batteries connected giving me 3.6kWh of energy if
required, though if you are planning on batteries being cycled often,
as would be the case in a renewable energy or CHP situation, I would
not allow batteries to be discharged more than 25% otherwise the life
time of your battery bank will be shortened.

As a test to ensure that the UPS worked as expected and to gauge the
run times of the batteries, I rigged up a 300w lighting load to the
UPS and left it going. It had done 8 hours by the time I decided that
it was time for bed, so I switched it off happy to know that it far
exceeds my requirements.

So far it has been used in power cuts and quite happily runs the
central heating boiler, house lighting (mostly energy saving CFLs) and
the TV/Sky box - so pretty much business as usual for us in case of
power cuts.

As others have mentioned, the Smart UPS is pure sine wave, whilst the
BackUPS is modified sinewave. Don't use BackUPS for anything with
Motors, nor CFLs - they may work, but at reduced efficiency and
reduced life span.

The Smart UPS series are line interactive which means they power the
load from the mains when it is with in voltage and frequency
specifications, and switch to battery when outside of specifications.
If you power the UPS from a mains generator you may find that it keeps
switching to battery instead of powering the load directly. An online
UPS may be able to cope with generators better as they rectify and
invert the mains feed, but they are more costly and slightly less
efficient. You will not have a problem running the SmartUPS from a DC
power source coupled to a Lister and connected directly to the battery
bank as the UPS can not tell the difference, and the batteries will
smooth the wave form.