What does that mean? Let's take the phone app part of out it, since it doesn't sound like that has anything to do with it.
"MaxPower" is an attribute of the device you are talking about? That attribute is getting updated by some event occurring within the driver?
If so, when you trigger the sendEvent, you would also want to update the setting using:
settingName, newValue and settingType would all have to be filled in with values or variables.
In this case, if maxpower is the setting and lets say power is the value and the type is number, you would use: