Not enough data to begin the design!
Here are a few questions to answer for starters. What's the input and output voltage? DC or AC? How long must the output be maintained when the input disappears?
Once these are known, the battery choice and circuit configuration can begin. The efficiency of the converter and how long they system lasts under full load will determine battery capacity -- though size, availability and monetary constraints must also be taken into consideration. Power input and battery type will determine the charging system.
While I've never personally designed a 2.5KW UPS, I've been an engineer long enough to know that fully defining the design constraints is the first (and often most difficult!) part of the job. After that, each piece can be tackled one at a time.
Good luck!
