I got a couple of POWERWARE/EATON 5125 Rack mount UPS units from work that are 1500WATT SINEWAVE units.   These have the internal battery packs that will run several things here in my radio/computer room for about three hours before going into low DC alarm.  These have the external 48VDC external DC POWER connectors on it and I was wanting to add some external batteries to it to make it run my room longer when we have power outages...   I don't think these units cost much on the surplus market...   I have clocks and fans and all kinds of regular 120VAC items plugged in the 120VAC power strips in the radio/computer room. Clocks never miss a beat,,,  Routers-Computer- monitors - HDTV and radio circuits don't even blink when it switches over... Just doesn't stay on for a long time.  Gives me time to normally shut things down I reckon...  
Another project to work on one of these days haha...    We used these at work to run our telephone/Internet interface circuits for comms gear... 
These being portable rack mount units sitting on my floor here I could drag them along in my OFF-ROAD trailer just fine.  they sure are a heavy thing however haha... Never done that...
Roy Ken