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Backup Mode vs 100% Self-Powered Mode

Eml2,

The Tesla FAQ states:

How does Backup work?
In the event of a utility power outage, the Backup Gateway immediately disconnects the home electrical system from the utility and restores power to the home. Whereas a traditional solar system typically shuts down during a grid outage to prevent any power from going back into the grid, because the Backup Gateway manages the grid connection, solar can continue to operate. Powerwall is able to coordinate its charge and discharge based on home usage and solar production, while curtailing solar production when necessary.

How does Self-Powered mode work?
In Self-Powered mode, Powerwall charges from the excess solar energy generated during the day, and discharges to power your home at night.

When solar is producing more power than the home is using, Powerwall captures and stores that energy. When the home is consuming more energy than solar is providing, Powerwall discharges enough energy to offset the home usage. If solar is producing more than Powerwall can store, or the home needs more than Powerwall can provide, the remainder is automatically exported or imported from the grid respectively.

In other words, Backup Mode reserves 100% of your battery for grid outages, where Self-Powered Mode allows the battery to discharge to a set reserve level for self consumption of stored solar energy.

If you are on a Time of Use (TOU) electrical schedule with your utility, a new Time-Based Control is in the works:

How does Time-Based Control work?
Time-based control makes use of an energy forecast to optimize the operation of Powerwall.

The energy forecast learns the patterns of your home energy use and seasonal solar production to maximize the value of your energy. If you are forecast to buy high-cost energy, Powerwall prioritizes charging during low-cost times and discharging during high-cost times.
 
In other words, Backup Mode reserves 100% of your battery for grid outages, where Self-Powered Mode allows the battery to discharge to a set reserve level for self consumption of stored solar energy.
But if the reserve is set to 100% in Self-Powered Mode, it means that it will not discharge any stored solar energy for self consumption, right? So, it is equal to Backup Mode?

What about the charging rate? I know in Self-Powered mode, each PW will take any solar left after house load, up to 5 kW. Is it the same as Backup mode?
 
Eml2,

Yes. In theory, if the reserve level was set for 100%, Self-power Mode would effectively equal Backup Mode. I am not sure why you would want to set the Self-Powered Mode to 100% reserve. It is easier to set Self-Powered Mode to say 30% and tap on screen between the two modes as needed.

From my experience, the charging rate (when solar is connected) is the same if the battery is full. If the battery in not full, there is a difference in charging priority (rate), per the Tesla FAQ:

Backup-Only
100% of your Powerwall is reserved for backup at all times
Charging Priority: 1) Any Solar
Discharging Priority: * Only during grid outage

Self-powered

Use stored solar to power your home after the sun goes down
Charging Priority: 1) Excess Solar
Discharging Priority: 1) Net Usage

Furthermore, the PW will charge at a lower sustained rate when there is no solar and the PW is charging from the grid. I recall seeing in forum posts in the 1-2 kW? range.
 
Last edited:
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cwied

Active Member
Jan 13, 2015
1,112
830
San Mateo, CA
Eml2,
From my experience, the charging rate (when solar is connected) is the same if the battery is full. If the battery in not full, there is a difference in charging priority (rate), per the Tesla FAQ:

I leave my Powerwall in self-consumption mode and just change the reserve percentage. In my experience, the "excess solar" only applies if the reserve is set to less than the current charge. If the reserve is set to 100%, all solar goes to the battery.

Here is the chart from this morning:
upload_2018-3-1_12-30-15.png
 
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cwied

Active Member
Jan 13, 2015
1,112
830
San Mateo, CA
In my opinion, a good starting point for the reserve setting is the one that leaves you with enough power in the batteries so that you can have power overnight if there's a power outage right after the sun goes down. Just figure out how much power you use from sundown to the point where solar exceeds your usage the next day.

After you try this setting out, you can judge to see if you're losing too much of the Powerwall's payback by using that reserve setting. If so, you could lower the reserve by trading off the likelihood of an outage against the extra benefit you get from the lower reserve.
 
GenSao

Eml2,

Yes. In theory, if the reserve level was set for 100%, Self-power Mode would effectively equal Backup Mode. I am not sure why you would want to set the Self-Powered Mode to 100% reserve. It is easier to set Self-Powered Mode to say 30% and tap on screen between the two modes as needed.

From my experience, the charging rate (when solar is connected) is the same if the battery is full. If the battery in not full, there is a difference in charging priority (rate), per the Tesla FAQ:



Furthermore, the PW will charge at a lower sustained rate when there is no solar and the PW is charging from the grid. I recall seeing in forum posts in the 1-2 kW? range.


Where did you get the table re charging and discharging priorities? My PW starts to charge and then appears to get distracted and moves to discharge under the net usage priority - this before the battery has fully charged with Solar.
 
Where did you get the table re charging and discharging priorities? My PW starts to charge and then appears to get distracted and moves to discharge under the net usage priority - this before the battery has fully charged with Solar.

The table was originally on the Tesla FAQ page on March 2018 when I posted it. Unfortunately archive.org does not have a cached copy.

The behavior you are experiencing is explained in the Timed Based Control FAQ:

Capture1.JPG


For Self-Powered mode, the Powerwall will use the Energy Forcast and aim to keep the battery level between you Reserve for Power outage (Say 30%) and Full (100%). After a set percentage (say 30% Reserve + 30% Energy Forecast) the Powerwall will start to discharge earlier to maximize self consumption.

A battery does not have to charge to 100% all the time. It is healthier for the battery to be somewhere in the middle.
 
I stumbled on this thread because I wanted to find out if there is a difference between Backup-only mode versus 100% self-powered mode.

My observations for Backup-Only mode is that the battery is charged from solar when it dips below 97.0%, and it will charge it up to 99.0%.

And for 100% self-powered mode, the battery is also charged when it dips below 97%, but it will charge it up to 100%.

I am not sure why there is a difference in behavior. I suspect that Tesla has optimized backup-only mode, and did not apply this to the self-powered mode with reserve of 100%.

I usually leave it on Backup-only mode for that reason. But, if there is severe weather, and I want to make sure I have the battery fully charged, I switch it to Self-Powered at 100% reserve until charging is completed.
 

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