Our Tesla Solar Home Update — It Just Gets Better & Better!

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As I write this on a Tuesday morning, we are just beginning our 5th week of operation of our Tesla solar array + Powerwall setup. I thought it would be a good time to share the operational experiences and learning curves we’ve gone through to optimize operation and minimize our exposure to grid-supplied power.

For anyone who hadn’t read my first article about the Tesla Energy experience, the short version is that we installed an 8.125 kW solar array on a predominantly south-facing roof on our 2800 sq ft colonial located in the woods 6 miles to the west of the village of Woodstock, VT. In addition to the array, we had Tesla Energy install a Powerwall 2 and a dedicated 48 amp vehicle charger.

The most recent Google satellite photo available does not show the actual panels in place, but the position of the sun when the satellite pic was taken gives you a pretty good idea of our exposure. The trees nearest the house were also removed to allow for a longer daily exposure through all seasons.

There was 48.8 kWh of solar energy production. We used 15.2 kWh of that as well as 17.2 kWh that had been stored in the Powerwall. See next image below for more details.

The Tesla app is your best friend for monitoring and controlling the operational parameters of the system, especially if you have a Tesla vehicle and Powerwall in the mix. The app lets you:

1) Toggle between your vehicle, the Powerwall, and the solar array to see the status of each in real time. It allows you to start and stop charging your vehicle (if it’s plugged into a charger, of course).

2) Customize your Powerwall. Here are the options:

Backup-only: 100% of your powerwall is reserved for backup power at all times.

Self-powered: Used stored solar to power your home after the sun goes down. You can set the reserve battery storage (for use during power outages) anywhere from 0 to 100%.

3) View usage and stored energy by the hour, day, week, or month.

The normal sequence of operations is as follows: Solar energy is converted into DC electrical energy through solar panel absorption. This energy flows to the SolarEdge inverter where it is converted into utility-frequency AC. The dwelling, as it may be, is the first destination to use this electricity. Anything generated beyond the required “at the moment” dwelling usage is directed to the Powerwall to charge it to 100% at a max instantaneous rate of 5kW. If there is still more power being generated by the array than can be used by those two destinations, the overage is sent back to the grid.

All of these values are constantly fluctuating dependent upon the amount of radiant energy getting to the array through the earth’s atmosphere. Even on the cloudiest days, our array will generate a minimum of 0.2 kW of solar at a time, which is enough to power our home when no one is home and energy usage is at a minimum.

The four screenshots included with this article are taken directly from the app. The 2nd and 4th, taken within one minute of each other on Wednesday, May 9, 2018, show the overall effectiveness of the system that is powering our house while also allowing me to charge the Model S for my daily commute, some days without using any external power sources.

The 1st and 2nd images show that at those instants, we were able to be self powered — that is, no input from the grid from 12:01 am those mornings until the time the photos were taken.

Over that 24 hr period, 42 kWh was used by the home. Remember, very little of that was grid-supplied power. That includes charging the Tesla Model S from 54% to 98% battery capacity. We used 0.3 kWh from the grid but sent 9 kWh back to the grid.

The 3rd photo (right above) depicts a charging car situation. The Powerwall had charged up to 100% capacity prior to me starting to charge the Model S. What you are seeing here is 7 kW being provided from the solar array with the balance of required power coming from the stored energy in the Powerwall. This is by no means the normal situation, but there have been several occasions where I am home midday or weekends and can achieve this level of self-sufficiency.

This final shot is quite busy and would take a couple of paragraphs to fully explain. The 4 icons depicted across the top are home usage (blue), solar production (orange), Powerwall battery (green), and grid (grey). Above the “x” axis is energy used by the home, whether in green by battery storage depletion, solar energy, grid energy, or any combination thereof. Below the “x” axis is energy stored or given back to the grid. The large multi-colored swatch midday was when the car was being charged, again not using any grid-supplied power. The orange parabola shows solar generation throughout the course of the day, with none being sent to the battery or returned to the grid while charging the car.

Finally, with the app, I can monitor and change parameters from any location at any time, including starting and stopping the car charging function as I see fit to utilize the most efficiently produced and financially beneficial energy over the course of the day, as long as the car is plugged into the charger.

We have yet to have our “Tesla Tech Talk” at the home to share our experiences with other potential solar converts. That will be the topic of my next article once it occurs.

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