Disclaimer: I am not a UPS expert. I have not researched about the working or a UPS either. The following is a collection of 'hear-say' material only. U r free to research and prove me wrong.
Here's some points wrt a UPS:
1. It is normally recommended NOT to connect one battery operated device to another. In other terms, a UPS is NOT recommended to be connected to an inverter. Perhaps (just my theory, i dont know the exact reason
) this has to do with the fact that most inverters output square-wave AC instead of sine-wave. (i know some give sine-wave o/p)
2. A higher rated UPS doesnt necessarily mean more back-up. the VA rating is the load capacity that the UPS will take. The back-up is determined by the battery thats packed into/along with the UPS. Here's an anology i can think of - Its like having a bike and car. The VA rating on the UPS is something like the seating capacity of the vehicle. The back-up time is something like the mileage/tank capacity. More the seating capacity, more load the vehicle can carry. Higher the mileage/tank capacity, longer it runs.
3. Also, going thru
APC - Country Selection Page the 1.5KVA UPS typically supports a max load of 865 watts. and at that load, it gives a back up of around 5 mins. :O So, with a 21" CRT, you can expect a load of ~400-500W, which translates to about 12 mins back-up. I dont know if i read it right, but I am quite shocked by those figures. Here's the link -
APC BACK-UPS RS 1500VA 230V INDIA
4. Typically, higher rated UPS have an option to connect an external battery for more back-up. I have seen a Powercom 600VA UPS which had an option to connect an external battery. This kind of solution would be ideal when you need more back-up, like in this case - 2 hours.
5. The APC 1.5KVA UPS at that advertised price of ~9.9k might not give you enough back-up unless you couple it with a good and high rated battery. If u add in the battery cost, the solution would come to around 15k minimum.