Switching-Off PWs Question...

[JayClark]

Is any one aware of problems with "Switching Off" the PWs? In my case over the weekend I'm considering switching them off after my peak period on Friday, at about 40-50% SOC, then turning them back on a few hours before the Monday Peak periods starts. The PWs definitely pay for themselves being used during the peak TOU periods based on my utility peak rates and demand charges and such.

However, my utility Customer Generation TOU plan is all off-peak on the weekend with very cheap off-peak rates, and I've measured the losses charging/discharging the PWs throughout the day, and then later while just "sitting" idle during off-peak periods, and the system seems to lose 4-5 of the 18-22kWhrs per day the PV produces typically, as compared to what I've measured when I've switched off the PWs for a day, and based on what my Utility reports each day.


I seem to come out up to 5kWhrs ahead on days when I don't need to manage and/or avoid house loads during Peak periods (and avoid Demand charges), and instead just let extra solar (after powering the home) just go back to my utility for a 1:1 offset monthly bill offset. I don't get paid for feeding back, but I get a offset from the kWh's I do use, plus then I don't lose solar to vampire drain and round-trip PW losses, and seems like it would be quite a few less net charging cycles (longer PW life maybe) on the PWs to have them shutoff for the weekend at a medium-to-low SOC.

Thoughts? Anybody else do anything like this?

 

[mspohr]

Is any one aware of problems with "Switching Off" the PWs? In my case over the weekend I'm considering switching them off after my peak period on Friday, at about 40-50% SOC, then turning them back on a few hours before the Monday Peak periods starts. The PWs definitely pay for themselves being used during the my peak TOU periods based on my utility peak rates and demand charges and such.

However, my utility Customer Generation TOU plan is all off-peak on the weekend with very cheap off-peak rates, and I've measured the losses charging/discharging the PWs throughout the day, and then later while just "sitting" idle during off-peak periods, and the system seems to lose 4-5 of the 18-22kWhrs per day the PV produces typically, as compared to what I've measured when I've switched off the PWs for a day, and based on what my Utility reports each day.


I seem to come out up to 5kWhrs ahead on days when I don't need to manage and/or avoid house loads during Peak periods (and avoid Demand charges), and instead just let extra solar (after powering the home) just go back to my utility for a 1:1 offset monthly bill offset. I don't get paid for feeding back, but I get a offset from the kWh's I do use, plus then I don't lose solar to vampire drain and round-trip PW losses, and seems like it would be quite a few less net charging cycles (longer PW life maybe) on the PWs to have them shutoff for the weekend at a medium-to-low SOC.

Thoughts? Anybody else do anything like this?

Interesting that the parasitic load of the Powerwall seems to be about 5 kWh/day. This seems very high. Have you asked Tesla why they use so much power doing nothing?

 

[JayClark]

Interesting that the parasitic load of the Powerwall seems to be about 5 kWh/day. This seems very high. Have you asked Tesla why they use so much power doing nothing?

I could be wrong, but I don't think it's actually high compared to other inverters based on my experience and calculations. I think it's probably better by a note worthy margin compared to the next best in the industry, again based based on just my experience and calculations.

I think, that if we say the PV system produces (for example) 22kWhrs during the day then the following is roughly occurring...

• Assume the PW is 95% efficient, that means of that 22kWhrs only 20.9kWhrs actually makes it into the PW storage. (Keep in mind, a standard PV inverter, is only reporting efficiency to turn DC power from the PV-crystals-sun into AC power - so a single inversion- and they do no power storage or distribution - so here we're not talking about solar inverter efficiency.)
• Then, to get that 20.9kWhrs back out later in the day to power the home, let's say again it's another 5% loss resulting in 19.8 kWh's returned to the home.
• Now say each PW uses about 30watts/hr being turned "on" (roughly twice what my iPhone peak charge rate would be), in order to be ready to respond to requests to charge/discharge, battery balancing, reporting status, managing schedules, cooling/warming fans, etc, that is 750Whrs used per day per PW, but then I have four PWs, so that adds up to about 3kWhrs loss to "operations".
• All of this totals about 5kWhrs per day, basically matching my actual observed system performance.

Actual specs and losses may be slightly more or less in different areas, vs what I've calculated here, but overall these calculations match what I've measured. For instance it's very possible the efficiency is lower, but then the "operational" kW costs must be lower too, vs what I've calculated - so that it all nets out at about 5kWhr one way or another in a typical 22kWhr solar day.

Most of the inverters, and battery storage systems, with which I've had experience directly and/or based on research and the reporting of other people, generally perform more poorly than their own marketing specs "ideal" scenario "potential peak efficiencies", and notably perform under these actual measured performance numbers that I'm seeing for the PWs consistently each day.

Regardless, my main take-away still seems to be that I can get back roughly 10kWhrs each weekend by simply having the PWs switched-off when I don't need them for those two days. Also, another takeaway might be, if you don't need 4PWs, get fewer to avoid the additional expected parasitic losses from having multiple PWs. In my case, in the winter I probably only needed 2 PWs to get through peak periods, but in the Summer here in Arizona I need (err... or wanted) four to get through peaks "comfortably".

Clearly if I have them switched off I don't have automatic back-up in the case of an outage, but those are rare enough in my area that I'm okay walking to the garage to turn them on during an outage if needed.

 

[power.saver]

You should probably just put them in Backup mode for the weekend if you don't want to self-consume or have no TOU. If you turn them off, and the power fails, they won't come back on until the grid is up.

 

[mspohr]

I could be wrong, but I don't think it's actually high compared to other inverters based on my experience and calculations. I think it's probably better by a note worthy margin compared to the next best in the industry, again based based on just my experience and calculations.

I think, that if we say the PV system produces (for example) 22kWhrs during the day then the following is roughly occurring...

• Assume the PW is 95% efficient, that means of that 22kWhrs only 20.9kWhrs actually makes it into the PW storage. (Keep in mind, a standard PV inverter, is only reporting efficiency to turn DC power from the PV-crystals-sun into AC power - so a single inversion- and they do no power storage or distribution - so here we're not talking about solar inverter efficiency.)
• Then, to get that 20.9kWhrs back out later in the day to power the home, let's say again it's another 5% loss resulting in 19.8 kWh's returned to the home.
• Now say each PW uses about 30watts/hr being turned "on" (roughly twice what my iPhone peak charge rate would be), in order to be ready to respond to requests to charge/discharge, battery balancing, reporting status, managing schedules, cooling/warming fans, etc, that is 750Whrs used per day per PW, but then I have four PWs, so that adds up to about 3kWhrs loss to "operations".
• All of this totals about 5kWhrs per day, basically matching my actual observed system performance.

Actual specs and losses may be slightly more or less in different areas, vs what I've calculated here, but overall these calculations match what I've measured. For instance it's very possible the efficiency is lower, but then the "operational" kW costs must be lower too, vs what I've calculated - so that it all nets out at about 5kWhr one way or another in a typical 22kWhr solar day.

Most of the inverters, and battery storage systems, with which I've had experience directly and/or based on research and the reporting of other people, generally perform more poorly than their own marketing specs "ideal" scenario "potential peak efficiencies", and notably perform under these actual measured performance numbers that I'm seeing for the PWs consistently each day.

Regardless, my main take-away still seems to be that I can get back roughly 10kWhrs each weekend by simply having the PWs switched-off when I don't need them for those two days. Also, another takeaway might be, if you don't need 4PWs, get fewer to avoid the additional expected parasitic losses from having multiple PWs. In my case, in the winter I probably only needed 2 PWs to get through peak periods, but in the Summer here in Arizona I need (err... or wanted) four to get through peaks "comfortably".

Clearly if I have them switched off I don't have automatic back-up in the case of an outage, but those are rare enough in my area that I'm okay walking to the garage to turn them on during an outage if needed.

Yes, that makes sense. I didn't take into account that you have four Powerwalls.

 

[nyprepper]

i have two PW's that I leave set at 100% reserve, backup only. I have about a years worth of data and the vampire drain on average is just over 2% of total capacity daily.... about .6kwh total. they charge up to 99% with solar, drain over the course of 24 hours to 97%, then begin charging again. this is based on the data I observe in the mobile app.

 

[KY-Lonewolf]

I just looked at my vampire drain for the last 8 days..........
Conditions:
4 PWs with version 1.41.2 located in an insulated garage.
The garage has been 52 to 64 degrees F with never dropping below 50 degrees
The PWs are set to Backup-only 100%
The PWs charge every other day and total 1.7 kWh per charge.
They charge first thing in the morning when the PVs start to make power
That's a drain of 0.85 kWh per day for all 4 PWs combined.

 

[JayClark]

I just looked at my vampire drain for the last 8 days..........
Conditions:
4 PWs with version 1.41.2 located in an insulated garage.
The garage has been 52 to 64 degrees F with never dropping below 50 degrees
The PWs are set to Backup-only 100%
The PWs charge every other day and total 1.7 kWh per charge.
They charge first thing in the morning when the PVs start to make power
That's a drain of 0.85 kWh per day for all 4 PWs combined.

Thanks for the data points since you have 4 PWs and very similar garage situation it makes for a good comparison, my garage may average about 10 degrees warmer right now. Sounds like I'm calculating too high for the "maintenance" losses, but then I must be undercounting the turn-around loses since I'm not in 100% backup, rather at this time of the year I am discharging to about 45% by the end of my morning Utility Peak period, and hitting a Peak charge of about 75% right before my evening peak period starts, based on the way I've set the shoulders. This weekend I'll test running in 100% backup, I'm guessing my numbers will match yours.

Is your calculation based on the numbers from the Tesla App, or do you have another way of also measuring power usage?

 

[KY-Lonewolf]

[QUOTE="Is your calculation based on the numbers from the Tesla App, or do you have another way of also measuring power usage?[/QUOTE]

Tesla App - "kWh to Powerwall" I have no "kWh from Powerwall" other than vampire drain which is not reported.
Only Home Usage is reported.
I am set to Backup-only 100%

 

[MorrisonHiker]

Tesla App - "kWh to Powerwall" I have no "kWh from Powerwall" other than vampire drain which is not reported.
Only Home Usage is reported.
I am set to Backup-only 100%

They removed the "to Powerwall" numbers when looking at non-daily data back in August/September when they updated the app.

You can view the "from Powerwall" if you are on the Energy Usage screen and scroll down below the graph. You'll see both "from" and "to" when viewing the daily data but only the "from" when looking at weekly, monthly, yearly and lifetime.