I have been using Sukam Inverter with 2 Batteries, for last 1 year now. I have Installed it in a small office I operate which runs 7 Computers & a Printer along with 4 Tube Lights & 2 Fans it has gives a decent power backup for 4-5 hours time.
Luckily till date we have not had an issue where the batteries have drained out completely so it is difficult to advice how much actual backup it provides. the best part is that the Inverter also functions as a UPS for all the systems. It also controls the voltage fluctuations and easily shifts from power to inverter without interfering for the systems.