I am not sure what ON/OFF would do. You can’t shut off/turn on the PV system via envoy. You have to do so physically using a breaker
I am not really sure how useful this would be as a smart device. One thought I had was that you could use it to turn devices on when solar energy production is high. Depending on your local power companies strategy for charging for power from the grid relative to put on the grid by your system this could be very useful. In Hawaii it will not be so useful at this time because the power company currently gives you equal value fro what to deliver to the grid and what you use. This may be changing. They want to change the model to one where you get paid 50% of the cost of a kilowatt hour for what to give to the grid vs what you take off the grid being charge at full cost.
This means it is better to run devices during the energy generation hours and if energy production is low due to clouds etc you may want to reduce the devices you are running.
So I see this as a sensor. Reporting energy production levels. Then I can have a smart app that says when energy production is low shut off water heater, AC Unit, etc. When energy production is high turn these devices on.
Mostly I wanted to see if I could write the device interface and learn from it
My understanding of the API is I can have 10 hits/min 10,000 hits per month as a customer.
So I think every user would have to create their own account.
If this was a profitable device type you could purchase the higher usage plans and let everyone use the same account. So providing a third party service interface could use this model.
Here are the detail of the plans.