APC says the batteries need to be replace every 2-3 years. I get 5-6 years, but don't lose power often or for long periods. A new battery is around $40.
A 400w UPS can provide 400w for 20-30 minutes. It won't power an 800 watt coffee maker during a hurricane (tried it), shuts down. Don't think it will let you exceed the wattage rating. Might get away with higher capacity batteries for more run time though.
I saw a hack once for increasing the output of a car inverter, by replacing the power mosfets, not sure if that would work in a UPS though...