It depends on the threshold and the resolution of the plug i.e. how many watts should differ for it to register as a change and how sensitive it is.
When I did my tests I connected a laptop charger to a plug which fluctuated 0.5w every second while charging, this together with 3 more plugs with different stuff connected (TV etc) bombarded to hub with several events every second which lead it slow down over the course of 24 hours.
The way you need to do it is to set a threshold either in percent or an absolute relative value for the hub to register a change in watt. If you are measuring a TV and are trying to figure out if it's on or off this shouldn't be a problem, I just wanted to know how much of a load the hub could handle before things got bad