I recently had to do this. The server I was putting up was rated for 3kW. To determine the expected load, I put it under a dummy load that I reasonably considered the maximum for what I would expect on the server (this was a dev machine, so I picked compiling the linux kernel as a benchmark). I ran that until the power stabilized (SuperMicro servers can measure power consumption in hardware and expose this via IPMI - very handy), because power consumption may keep creeping up for a few minutes as the fans adjust to the new operating temperature. I then repeated the same exercise with a CPU torture test (Prime95), just to see what the maximum I could possible get out of the machine was. The numbers turned out to be about 1.8kW for the linux kernel benchmark, and 2.3kW for the torture test. What I ended up doing was to provision for 2kW and use the BIOS' power limiting feature to enforce this. That would kill the machine if it exceeded its designed load, but that's usually better than tripping the breaker and killing the entire circuit. You may also want to talk to your data center provider about overrages. Some of the quotes I got wouldn't kill your circuit when you went over, but would just charge you (a crazy amount, but better than losing all your servers).

Hope that helps. Your deployment is larger than ours, so there may be other techniques, but that's what we did.