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How to efficiently charge your EV at home

I do most of my charging (90%) at home. Therefore, I thought a lot about how to most efficiently charge the car. My efficiency considerations are in three areas (there are probably more, but I focus on those for now):

  1. Use the lowest cost of energy $ per kWh throughout 24 hours: Dependent on your location and the utility company you are using, there might be different $ per kWh rates throughout the day or night. Also, some utilities offer special EV rates. Check with your utility to see what you can do to minimize charging cost.   
  2. Use the "cleanest" energy: Since we have solar and home battery, I try to use as much energy as possible from our solar panels to charge the car. In addition, you should avoid charging at high electricity demand times (in our area 3pm to 9pm)  because then the utilities use "peaker plants" to stabilize the grid, often using "dirtier" energy. Also, Tesla now offers an option in selected location to charge from solar only. Check out their website for more info.
  3. Minimize "waste" of energy: The waste of energy is probably minimal, but I still find it interesting to explore. I see two sources of potential "waste" when charging. First, the electronics in the car use energy to manage/process the charging session - let's say the car computer uses 150w per hour. Therefore, if you charge at a very low rate, maybe 1kwh, then 150w of the 1kwh are used just for running the system. If you charge at 6kwh, then the 150 w are a smaller percentage. Therefore charging on 120V seems to be more inefficient than 240V. Again, I have not calculated this in detail, but I think it is something to consider. Second, since we have solar, I try to use the solar energy produced to feed right into the car, versus storing it in the home battery and then charging the car later. Storing and converting solar energy in a home battery and then getting it back later can waste 10 to 20% through inefficiencies and AC/DC conversion (roundtrip efficiency). 
Tesla EVs offers a convenient way to schedule the charging session - you can set the time, the charging level and also the charging speed - all from the app. With that in mind, it should be fairly easy to charge your car as efficient and clean as possible based on your home set up (solar, battery) and the electric utility services in your area.





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