If you want to connect sensitive equipment like computers and mobile chargers etc with your wall sockets that are on UPS, you should avoid purchase of the "Square Wave" UPS that are available in the market. You must have noticed the increase in noise being produced by ceiling fans, etc when they run on the supply from these boxes.
Square wave AC inverted from these UPS hinders the normal function of AC equipment that has motors and transformers having Copper windings, subsequently damaging them in short term use.
Budget will be an issue for you, but if you go for a refurbished 1kVA 12VDC Pure Sine Wave APC UPS available in the market or an Off the Shelf Near-Sine Wave UPS atlest, these will cost you around 15 ~ 17k. Battery, yes that will be an add-on. For that, I'd recommend a 100 AH SLA battery (preferably AGS). This will cost you around 5 ~ 7k.
Backup time calculations (approximates):
1kVA = 700 Watts
Keep 20% of your available load as reserve.
80% of 700 watts = 560 Watts
Line losses for internal wiring = 10%
10% of 560 Watts = 56 Watts
Available load capacity = 500 Watts (approx)
Battery capacity = 100 AH
Current draw from battery if full load of 500 Watts is applied = (500 Watts / 12 VDC) = 41 Amperes (DC)
Time (kWh) = 100 AH / 41 Amperes (DC) = 2 hrs 30 mins approx ..
