We are building a new church which would seat about 1500 . We already have a PA system of 2,700 W. Our program mainly consists of vocals and light music (keyboard, drums). Do we have enough wattage for 1500 people? Is there any approximation/ rule between number of people and wattage? The hall is rectangular 50 X 100 feet with a gallery of about 70 X 50 feet.
What you have just asked is literally equivalent to "I have to haul 5 tons of gravel, my truck engine is 240 horsepower, will it work?
Wattage is a measure of power, not a measure of volume or sound pressure level. There are too many other factors involved (including speaker sensitivity, measured frequency bandwidth, construction, shape, dimensions, acoustical profile of the room, etc) to make it a meaningful measurement of much of anything in installed sound applications. There's really no way to answer your question without a LOT more information about a lot of different things.
If you're building a 1500-seat church, you are engaged in a project of enough scope and budget that you should engage a competent system designer, because if it were simple, we designers wouldn't exist. Think of the designer as the AV equivalent to your building architect- most people know a thing or two about building a wall or putting in a sink, but it takes a professional to know how to bring all those pieces together into a successful whole.
It's possible some of your existing system can be re-integrated successfully, but once again, it's hard to tell. Please be a good steward of your congregation's trust and investment and get a professional opinion from someone who does this every day as their main line of work.
Michael Hoddy is right. It is not more watts to make it go farther it will be different cabinets or combinations on delays to get it where it needs to be. Your current amps may well work fine but your speakers may or mat not be useable.
Get bids from well respected sound companies in your area and make them win the bid to keep cost competive. Take a road trip to local and regional churhes simular in size and shape and make calls to their references to see if it sounds good. the travel money could save you much more than fixing the wrong install.