Hello Naresh,
This EngineerZone forum is for applications assistance of Analog Devices, Inc. products -- for example, their energy-monitoring integrated circuits used in electric meters. That being said, I'm so sorry you're experiencing this problem -- and there are several possible solutions, depending upon your expertise and what's available locally.
If your computer is a laptop, you're halfway there (most of them can run four hours on their own battery). Also they use far less power than a desktop tower and monitor, making the power source you need much simpler and less expensive.
You could use a 12-volt sealed lead-acid (SLA) battery of about 18 ampere-hours (AH) capacity. Charge this overnight where AC power is available with a small automotive battery charger (2 amps will do, these are available as a wall-wart like cube). In the office, connect the battery to run a small 12V, 100-watt inverter that normally plugs into a car's cigarette lighter. Add a 10-amp inline fuse in series with one of the battery leads as close to the battery as possible. This should operate your laptop (through its usual AC power adapter) for about four hours; you'll run on internal battery the rest of the time. Be careful -- sealed lead-acid batteries are generally safe but can leak or rupture if overcharged, connected backward, or short-circuited. I recommend this method only if you (or a friend) are experienced with electricity / electronics.
There are portable battery packs on the market that combine the battery, charger and possibly the inverter -- find what's available locally via search engine. A ready-made solution like this will be easier and safer.
Best regards,
Bob