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System switching to backup power while still drawing from the grid

System switching to backup power while still drawing from the grid

My system was installed and turned on in June 2019. It is a 15kW system made up of 41 Tesla panels. Roughly half of the panels are on one string that connects to a Powerwall, and the other panels on a separate string that only power the house when it is day light and connected to the grid.

I live in Pittsburgh, PA and I have a heat pump. In the summer on normal day the Tesla app tells me the AC energy usage is about 2.5kW when it is on. As the temperature cooled going into the Autumn and the heat pump switched over to heat, the energy usage was basically the same.

At some point the temperature gets cold enough that a temperature sensor tells the heat pump to cycle into a defrost mode. It is a normal operation and mine that mine has done when the temperature dips for the last 5 years w/o any issue. I have observed, through the Tesla app, that when when the heat pump cycles into and out of defrost mode the energy usage will surge to between 13kW and 15kW, from 2.5-ish kW, for a short period of time. Maybe 10-15 seconds.

Okay. That is the background. Here is the scenario when I encounter the backup mode problem.

my wife’s morning routine sometimes consists of putting clothes into the dryer.

My wife wakes up and puts clothes into and turns on the dryer. The dryer is drying, the heat pump is heating, the heat pump’s temp sensor is sensing. Because the temperature is dipping into the 20s in the early morning hours the heat pump is cycling into defrost mode and with the drying running and using around 4.5kW, my home’s energy usage is around 18kW.

This condition causes my system to go into back up mode. I’ve experienced it, and the Telsa app has recorded it. This has happened 19 times across 5 mornings since November 13th. The time on backup power ranges from 4 seconds to 10 minutes. The 10 minutes is an outlier.

I could be wrong, but this shouldn’t happen. The grid should supply what the house needs.

I think it is fairly safe to assume that the heat pump is operating the way it has been in the past, and that something with the gateway isn’t functioning as intended.

Any similar experiences or thoughts as I wait for Tesla to send out a tech? I called about 2 weeks ago.

daveandheidi | December 7, 2019

UPDATE: Right now the house’s energy usage is 17.3kW, mostly caused by auxiliary heat and the dishwasher, and the system is not in backup. 14.4kW from the grid and 2.9kW from the panels. The difference in this scenario is that the panels are supplying energy whereas in the above scenario they were not.