How can I determine the peak power consumption of my office
Simplest solution: Kill-A-Watt power meter.
Plug in your power strip to it and let it calculate the power consumption for you.  The best part is that it has direct carryover and will let you monitor power usage of other devices that you don't have nearly as much info on as you do your computer.
As far as theoretical peak comsumption, and subsequently the hourly cost:
Peak power consumption: sum of power draw of devices
Hourly cost at peak usage: peak power consumption * hourly rate
So, for your case: the peak power consumption would be 1200W + 110W + 110W + 300W, roughly.  That comes out to 1,720 watts. That would be 1.72 kilowatts.  So, multiply that with your hourly power rate, and you have a cost of ~$0.19/hour to run your system at full bore.