Desiree, here is an example for a UPS system with extended runtime.
You can add more runtime by adding more external battery packs (up to 10)

APC Smart-UPS XL SUA2200XL 2200VA 120V Tower/Rack Convertible UPS System

APC SUA48XLBP Smart-UPS XL 48V Tower/Rack Convertible Battery - 1 to 10 of these - 184 lbs per unit

or alternatively one BIG external battery for about 5 hours at 700W load:
Smart US48VDC Ultra Battery Pack (280 lbs)

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GR - 11-Feb-2013