what is the total load of all the items in KWh you want to run off the inverter? If you have an electronic meter (1-3 phase) you can see the load on the meter itself.
ex: 1 pc with led monitor would be around 150-200 watts. 15 of these will be max 3000 watts (or 3 KW)
Add to these fans, lights, printers, and other things. The wattage adds up pretty quickly. By a simple calculation, you will need atleast a 7-10 KVA backup. Lead acid batteries are not suitable for such heavy consistent loads. They will provide power for a couple of minutes max. Get tubular batteries since they are meant for deep discharging. There are various AH capacities and higher you go the price increase drastically.
You'll need to run the UPS off a diesel generator within minutes of the power going off.
If you're looking at a single UPS for the entire office talk to an expert from APC or some other company like TATA Liebert.
If you're looking at simple method, then get a 600VA ups for each of the pc's. Then get a generator with enough power to run your entire office. The UPS will stop the comps from going offline (around 20mins) till the generator gets online. Its best to put a mainline voltage stabilizer to remove most of the voltage fluctuations and surges that might come.
This is just my idea. Best to talk to someone and take 2-3 opinions before you do anything.
