ReadyNAS 516 ReadyNAS4220X

Well, that's going to depend entirely on what models and number of drives are installed and is going to vary wildly depending on the ongoing activity of the units.  The only way you could get a reasonable answer for your specific installation is to install equipment that will monitor the power over some sample time. A "Kill A Watt" meter is a very simple device that gioves a very simple answer.  There are devices that collect periodic data that provide a lot more insight.

Quite frankly, that likely applies to many devices in your server room, unless you are looking for max potential values instead of real life numbers.  Are you sizing breakers, cooling, or what?  For breakers, you want max.  For cooling, it gets a lot more complicated.  If you go with max load for cooling, you'll overkill it by a lot.  But if you only use typical without accounting for peak times, you'll have too little.  Max of a walking window average is a good way to go, but can only really be measured, not calculated.