There are some great answers already that show you the average load while doing some common tasks, but if you're doing a power budget, you really only to make sure you have enough for the maximum load.
What you need to know are the operating AC voltage and the maximum draw in Amps. These can be found in either the technical specs and may be printed on the power supply itself. You get the wattage by simply multiplying these two numbers.
W = V(AC) * A
The converse is also true: if you have a 400W power supply, it draws ~3.6A (400W/110V)
As an example, on the power brick of my Dell mini I see 110V (1A), so I'm looking at 110W of power. Thats the theoretical maximum load. I'll most likely use less than this, but not more according to most electrical codes.
Let's take another example, a Dell 1905FP 19" Flat Panel Monitor. A bit of Googling will get you to the Technical Documentation page. Scrolling down to the electrical section shows the following:
AC input voltage / frequency / current 100 to 240 VAC / 50 or 60 Hz + 3 Hz / 1.5A (Max.)
Standard AC voltage in the US is 110V, so you're looking at 165W max (110V * 1.5A). That page also shows that the normal power consumption is between 32W and 65W.
To figure out your actual power budget, spec out an average system and find out the maximum power load for that then multiply by the number of systems you expect to have. That's your power budget.
Now, you can either get a generator that will power all your devices at maximum load if uptime is a priority, or get one that will handle 50-75% maximum load if price is a concern. As you can see from that Dell Monitor, most devices will operate at 30-50% of maximum load under normal conditions.