Calculating how Much Power is needed For Servers

How do I calculate how much power I need for my servers / switches / routers? This is needed when moving into a datacenter.

I figure I should just add up the wattage on all the power supplies even though all that power is not used most of the time, is that right? If there are redundant power supplies, should I count just one or both of them?

APC has a pretty decent power calculator here. It's certainly better than pen and paper calculations.

You will grossly overestimate using this method. We found dual socket servers with 600W P/S draw steady-state around 180W and dual quad-core SLI graphics workstations with 1000W P/S draw less than 300W most of the time. Properly designed redundant power supplies add very low overhead, just a few percent.

Buy a $30 power meter such as the Kill A Watt, leave it plugged in for 24 hours each for a representative device, and know instead of guess.