Hey Brian, there is no need to guess about these things in today's information age.
Here is a UPS calculator I often use for PBX gear. Choose the middle column and ignore that it says it is for Telecom. Just choose the [Configure by Load] button and enter in the total amount of wattage for all your gear totalled together and then the other preferences you can pick.
It will result in an overkill model from their lineup but you can compare the stats of the model with other units you are considering. I like it because at the bottom of each model it shows a graph of the % of total utilization you are using.
UPS Sizing Selector Here is another that simply shows the suggested minimum size you need. Just ignore the labels on each row. You can put the wattage for any device into any row and it will still be the same total.
A simpler one. 
