Synchronizing car charging with solar Output

My wife recently read Ashlee Vance's book about Elon Musk.  She found it compelling - and reported I would as well.  She was right.  I loved it.

As a result of learning more about Musk, his vision, and his drive we decided to order a Tesla.  Delivery is a ways out (Mar 2022).  But that hasn't stopped us from thinking about what driving will be like with an electric vehicle, and figuring out ways to use it as efficiently as possible.

Since we already have solar panels on our house, we started thinking about whether we'd need additional solar production, as well as strategies for optimizing charging the car.  It would be great if we could automatically adjust car charging based on  electricity being produced by our solar array and electric consumption in the house ... all in real time.  We want to see how much power we really need to meet our needs.  

Monitoring Solar Output
A number of months ago we decided we wanted a better real-time handle on whether we were sending power back to the grid or pulling power from the grid.  We bought an Eagle-200 from Rainforest Automation.  This little box (~$100) connects (via Zigbee) to your smart electric meter.  There is a nice phone app that provides a view of what is happening at the meter.  This enables us to see if power is being sent to the grid (solar production exceeding current household use) or the opposite - we're pulling power from the grid.​

Eagle-200 by Rainforest Automation

Eagle-200 App Display. Left image shows sending power to the grid. Right image shows pulling power from the grid.

Controlling Car Charging
The Tesla's phone app enables the user to start and stop car charging, charging based on time of day, and also charging speed.

Putting Them Together
The easiest solution to synchronize the two is to look at the Eagle-200's app and see if excess power is going to the grid, and then use the Tesla's app to start or stop charging as appropriate.  While this method clearly provides benefit, it's not ideal.  Weather changes through the day impact solar production.  Usage in the home also varies.  This results in sometimes significant variations in how much energy is being returned or pulled from the grid at any given moment.  An automated solution would be much more effective.

​Fortunately both the Eagle-200 and the Tesla have APIs that can be used to programmatically communicate with each device.

Since I have the Eagle-200 I was able to work out getting instantaneous power data from it into a Python program.  I wrote a simple program to start/stop car charging, as well as adjust rate of charge based on power flow at the meter.  Obviously I need to actually have the car before the solution is truly implemented.  But placeholders have been created where car-specific code will be placed when we take delivery of the car and the connections can be tested.  (I'm also confident the code, as written below, won't behave as expected when it really gets connected to the car.  I always surprise myself with unexpected realities when testing software.)

Hopefully a simple solution like this will enable us to maximize the utilization of power being generated by the solar array.

Thanks to Lalit Pandit for help with JSON and Python!