Use a Double battery UPS. Microtek is the one I am using. Also, using a LCD monitor will increase its back up. - Digital -
Well I am using the same..Microtek Double battery, Its used to give 30 mins backup for 3 months now its giving 15 mins.
its very much simple that with the time being backup time will be lower. Coz, battery becomes weak. but still you can increase backup time by running single program at a time, use small size LCD monitor, turn off speaker if not necessary and if you need long backup for emergency you can turn off ur monitor to get your job done if visual is not important in that time.