The Samsara Dashboard supports the use of Formulas to sum the values of multiple power meters in order to see a graph of totaled power usages across a floor, building, site or other collection of power meters.
Formulas are created at the Organization level, by clicking 'Add a Formula' on the Formulas tab.
Use the Formula creating tool to name the new graph, select the group you'd like it to appear in, and add the different power inputs that you wish to sum. You can select total power for each input, or select individual phases in the case of a three phase system.
Once saved the the Formula will appear as a new graph in the group specified.