A look at the production and finances of my Tesla Energy system

There are so many factors that affect how a solar energy system will perform and behave, including latitude and longitude, house construction, house orientation, roof shape, trees, neighboring buildings, nearby mountains, HOA, utility rates and regulations, family size and habits, whether you drive an electric car and how much you intend to charge at home, and so much more. Your system and factors will be different, and there’s no way I can provide information that covers all cases. What I can do — and will focus on — is sharing details of my setup. Hopefully that’s enough for you to figure out how that applies to you!

About our system and the data analyzed

• Time period analyzed is 2021-11-03 – 2022-10-29, the first whole billing year that I have data for.
• Our PV system is rated for 9.41 kW, but effective output maxes out at about 6 kW.
• Our solar production during that period was 9,555 kWh (average 26.5 kWh / day)
• Our consumption was 8,194 kWh (average 22.7 kWh / day, or 950W average instantaneous use)
• We have one Powerwall. When the red line is above zero and the gray line below zero is roughly where we might benefit from a second Powerwall.

Seasonal Behavior

• From February through September, there’s enough solar production to power the needs of our house and car.
  • However, with just one battery, during the months of February and September, while the PV produces more energy than is used by the home and fill the battery (so some energy goes to the grid), the battery capacity is not enough to make it through the night without pulling from the grid. So, a second battery would help during these two months.
• It’s from about mid-March through August that we rarely pull from the grid.
  • That is, during this time, the battery collects and holds enough energy from the day to power our needs at night, and we would not really notice if we were disconnected from the grid for those 5-1/2 months.
  • Because there’s excess solar production during the day — more than enough to power the home and replenish the battery — this is when we charge our car (i.e., charge before sending excess to the grid). If we charged the car at night instead, it would deplete the battery and cause the house to pull from the grid (and reduce backup capacity).
• From about mid-Oct until Feb, there is not enough solar production to power the house and charge the car.
  • For much of that, there isn’t even enough to power the home during the daytime; i.e., the battery isn’t very useful for shifting usage of production from daytime to nighttime.
  • So, during this time I set my battery to mostly be power-outage reserve (60-80%). That is, if there is enough surplus production during the day, only 20-40% will be used to help get through some of the evening.
  • So, the house mostly pulls from the grid at night.
  • During these months, I prefer to charge the car at night for the benefit of the grid/environment. It doesn’t change how much we pull from the grid, but when. Not charging during the day allows the battery — if it charged during the day — to help reduce peak usage (early evening).

Energy Costs & Savings

• Our utility paid about $0.07-$0.09/kWh that we pushed to the grid, while charging about $0.11-$0.29/kWh that we used (depending on usage tier; also, they changed the rates during the year).
  • So, even though we pushed more electricity to the grid than we used, grid connection and usage resulted in net charges over the course of a year. From April through August we accumulated some credit, but quickly used it up and returned to net charges in September.
• Our average monthly electricity bill before solar: ~ $145/mo
• Our average monthly electricity bill with solar: ~ $20/mo

So, our average electricity bill savings: ~ $125/mo, or $1,500/yr

• 12-month average cost per kWh: $0.0290 (the total amount of electricity we used from any source, divided by the net amount we paid to the utility).
• That electricity bill includes charging our car, so we’re paying about $0.0078 /mi (whereas a car that gets 50 mpg and paying $4/gal is paying about $0.08 /mi (10x as much). More on this below.

Long-term financial

• After tax credits, the solar glass roof will pay for the new roof (equivalent), solar, and battery in less than 30 years.
• If we subtract the estimated cost of a new roof (what we would have to do anyway if we didn’t get solar), then the savings pays for the solar and battery in about 13 years.

Tesla Model Y

• Most of our charging is done at home, where the effective cost per kWh is $0.0290 (see above)
• The lifetime efficiency of our car has been 3.75 mi / kWh.
• So, our cost per mile is about $0.0078.
• If we drive 500 mi per month, our savings would be about $46/mo over a car averaging 40 mpg @ $4/gal.

Energy usage

• Our baseline is about 400 W (stuff that’s running all the time, like computers, fridges, and other stuff)
• Excluding charging the car, we go as high as about 2,000-2,500 if we using a power-hungry appliance such as the A/C, hair dryer, or toaster.
• The 900 W instantaneous average includes charging our car, which could result in a draw as high as 5,000-10,000W depending on how fast we want it to charge (and if we’re running power-hungry appliances at the same time).

Driving cost and efficiency

• For comparison, here are costs per 500 miles (3.75 mi / kWh):
  • $ 3.90: Our cost charging at home with solar
  • $ 15.65: Charging at home using our utility’s lowest rate ($0.117/kWh)
  • $ 30.00: A 50 mpg gas car with a cost of $3/gal
  • $ 38.17: Charging at home using our utility’s highest rate ($0.268/kWh)
  • $ 60.08: Charging at an expensive Supercharger, about $0.45 / kWh
  • $ 80.00: A 25 mpg gas car with a cost of $4/gal