UPS units can certainly be used to power lighting or other small loads in case of powercuts, but as others point out they may not be the best solution.

Some UPS units do indeed require a grid supply to start, these could be used for backup lighting, BUT COULD NOT be shut down (to conserve battery power) and then re-started.
APC units CAN be shut off and then re-started.

If a UPS is used at one qaurter of it rated load, the run time will be more than qaudrupled, since the battery is more efficient at the reduced load.

If however the load is very low, say 2% of the rating, then you WONT get 50 times the run time. Thats because the internal circuitry of the UPS consumes some current, no matter how low the load.
The run time is unlkely to exceed 12 or 24 hours even with no load att all.

With basic electrical knowledge one can fit an external battery of much greater capacity in order to extend the run time, if you do this, remember that the invertor was probably only designed to run at full power for a few minutes.
To avoid overheating, I would limit the load to 50% of the rating if useing an external battery.
Also remember that there may be NO ISOLATION between the battery connections and the grid supply. There is therefore the potential for a fatal line voltage shock from the battery terminals. These must be regarded as line voltage connections and protected against being touched.

I recently installed a 2,500 VA UPS at my mothers home, this is fitted with a large external battery and powers essiential lighting, fridge, freezer, TV, cellphone charger, and base unit for cordless phone.
With everthing turned on at once, the run time is 6 hours, in more typical use it is about 24 hours.