The SUM() function is used to sum up the values of a similar variable name from multiple devices, optionally in a particular device profile.
Usage:
SUM('variable_name', 'profile_name') - see below for description.
SUM('INC1_AE','Inverters')
Return the sum of all variables with the name INC1_AE with a device profile name of 'Inverters'
SUM('watts')
Return the sum of all variables called 'watts' for all devices that the user has access to