Here is APC's runtime charts. If you have another brand in mind feel free to google for their runtime charts.
your load is 100w. Remember its not the size in VA that gets you 2 hrs, its the size of the battery inside. The 500va is how much it can power. (in this case 500va~300w)http://www.apc.com/products/runtime_for_extendedruntime.cfm?upsfamily=165
This one appears to be cheapest from APC.http://www.apc.com/products/resource/include/techspec_index.cfm?base_sku=SUA500XLJ