UPS probably would not be a good solution because the higher power ones need a higher voltage input. e.g. 120vdc for a 3kw UPS. Also, many UPS will not start without detecting an AC line input first. The loss of AC starts it.
It doesn't necessarily have to be heavy. The power conversion can done at high frequency with small magnetic components, so there is no need for a 60 Hz transformer, which would be heavy.
If you use more than one, it would be handy to have sync capability. Sync can be used to parallel or series their outputs for more current or voltage. Two 120VAC inverters can be used to make 240VAC split phase.
Check here for some options:
http://www.nooutage.com/They appear to have some good quality units from several manufacturers, so it is a good resource for comparing prices and features.
Maybe you can use a modified sine inverter for some loads and a lighter duty sine inverter for the loads that need it. Modified sine inverters are cheaper and more efficient. But some loads, like induction motors, are less efficient and run hotter on modified sine.
And in the future, when you get a hybrid or electric car, check out
http://www.priups.com/