Lessons Learned From 6 Months With The Tesla Powerwall

For those of you who have read my other articles, you know we had a Powerwall 2 installed with our solar generation system back in April this year. I was asked to provide my take on how things are going. What was expected, what was unexpected, how it’s been operating overall?

The short answer is GREAT! Here are the details: Tesla chose to mount our Powerwall in the garage and under the solar inverter. Very clean installation overall. I realize all installations are different and the unit is built to be mounted outdoors as well. However, for all of you folks in the colder climate regions I recommend an indoor install if at all possible. One thing I hadn’t thought of until writing this now is wouldn’t the wall storage degrade in the cold weather when not charging, just as the car battery does? A question for the experts I suppose.

Best rate of charge to full capacity we have experienced with a full sun day starting at a 5% capacity base was 3½ hrs. This is on an 8 kW array with the house using a fraction of a kW. This will change for us as the seasons are changing.

Although I expect that even on cold winter days if the sky is clear and we have a full sun late morning, early afternoon we can still expect to reach full capacity. I don’t know how people are obtaining their Powerwalls. Whether they are leasing or buying. We chose to buy ours as part of the overall package. This gives us “full functionality” of the unit.

I can simply dial in the percentage of overall use or overall storage I want in the battery daily. A change made today will affect operation within a 24 hour period. Since I supplement charging our Model S with stored battery capacity, I run it down to 5% each night. Recently, I have limited the minimum capacity to 15%. When the winter months take hold, I will probably raise that further to maybe 50% and if a storm is predicted even 100%, thereby giving us the maximum time of use if a grid-imposed outage occurs.

As mentioned, we have an 8 kW solar array. Since we have the Powerwall operating parameters on the Tesla app now along with the car, I am able to monitor the capacity on the wall pretty much instantaneously.  Charging our Model S with the array provides approximately 7.5 kW and the battery discharging at a rate of 5 kW without using any grid power. I have charged the car from 20% full to 90% full, and when the car is done, the battery will recharge to full. The car has a 75 kWh battery.

When charging from the grid plus the battery, the charging rate will remain at 48 amps.

The limitations are with the utility transformer. When charging directly from the grid without battery boost, charging is limited to 36 amps. Apparently, the limitation is the cabling from the line in from the grid to the transformer. There is no difference in the cable used for a 100 amp service panel and a 200 amp service panel. We have 200 amp service. Not too many homes are using high amp-drawing equipment. That is, until now.

Transfer to backup power during a utility blip is seamless. We’ve actually had at least half a dozen instances where the grid has failed and we’ve gone to battery backup. We didn’t even know it occurred until I checked the app. However, returning to normal operation can take a bit when line frequency waivers. The solar inverter seems to want to see 60 Hz before you’ll see return to normal operations on the app.

That’s about it for now. I’ll be writing another article later this month about the economics of our first six months of operation. Stay tuned!