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How do I get this behavior?

MJ_CA_2019

Member
Aug 19, 2020
107
26
Central CA
I want my 2 powerwalls to charge to 100% but only if there is excess solar production.
If my home usage exceeds the solar production I want the excess to come from the grid.

Is this possible?

Example: In Self-Powered setting with 25% reserve

4.3 kW Solar
3.3 kW Home

1.0 kW going to the PW

If my AC cycles on then I get

4.3 kW Solar
6.8 kW Home

2.5 kW from PW to home

Advanced Mode (balanced) with 25% reserve & Backup Only have the same behavior

100% of the Home load comes from Grid
100% of the Solar production goes to Powerwall

I'm testing this on a Saturday (all off-peak) if that matters.

Thanks!
 

miimura

Well-Known Member
Aug 21, 2013
6,165
5,761
Los Altos, CA
When you're in Advanced - Time Based Control (Balanced or Cost Saving) the behavior will be different based on which time period the system is in. During Off-Peak the PWs should charge from All Solar with the house powered from the grid. It will continue like that until the PWs get close to full or the system thinks it will be full before the next period change. Then, the PWs may charge only from Surplus Solar with the grid approximately zero in or out. After the system has been running for a while, it may do things you don't want during Off-Peak weekends that have no other TOU period. You may want to put a fake short Peak period just to force it to charge if it doesn't.
 

BrettS

Active Member
Mar 28, 2017
2,109
2,512
Orlando, FL
I’m having a hard time imagining how this would work. Exactly when would you use the power from the powerwalls? With your method the powerwalls will charge with excess solar power, but when your house is demanding more power than the solar can provide it will draw from the grid so the powerwalls will never discharge. They will get to 100% and then what?

In self powered mode it prioritizes using the power in the power walls over the power in the grid. It will only pull from the grid if there is no other option because that would not be self powered.

With the advanced time based controls it will prioritize using the power in the powerwalls during the peak usage times so it can avoid pulling from the grid when the power is expensive. During other times it will allow the powerwalls to charge and allow power to be pulled from the grid.

But I’m just not sure what you are hoping to achieve here.
 

MJ_CA_2019

Member
Aug 19, 2020
107
26
Central CA
The thought was to try to avoid unnecessary non-bypassable charges.
The system seems to be prioritizing getting the battery to 100% rather than taking as little from the grid as possible.

Again, recall that today (Saturday) is all off peak. So with today, Sunday, and most of Monday before the next peak period, it doesn't seem to be a rush to get the battery charged to 100%. My reserve % is 25%.

Maybe part of my confusion here is understanding how the Advance (balanced) system is trying to function.
Is there any reason for my battery to have more than the amount of estimated peak usage + my reserve %?

It seems like if advanced mode consistently prioritizes charging my battery to 100% even when my battery will not get down to my reserve % by the end of my peak time, then it is taking more from the grid (and thus making me incur more non-bypassable charges) than necessary.

Thanks for reading and replying. I'm certainly open to a different way of thinking if I'm off here.
 

getakey

Active Member
Jan 28, 2020
1,115
359
95762
I would think charging your battery to 100% during Off Peak would be more cost benefit than worrying abut non-bypassable charges. That gives the greatest rate arbitrage during Peak. Personnally, I want my PWs to get to 100% every day even if they don't discharge to resserve by end of Peak.
 

MJ_CA_2019

Member
Aug 19, 2020
107
26
Central CA
I would think charging your battery to 100% during Off Peak would be more cost benefit than worrying abut non-bypassable charges. That gives the greatest rate arbitrage during Peak. Personnally, I want my PWs to get to 100% every day even if they don't discharge to resserve by end of Peak.

I think to some extent that seems to depends on the capacity of your battery system and your peak usage.

For example, on Friday my peak usage was 24.7 kWh and I produced 6.4 kWh from solar during the peak period that was exported to the grid. My battery (3 PWs) started the peak period at 100% charge and discharged 28.9 kWh all the way down my reserve capacity of 25%.
So the system put more in my battery than was needed to cover the peak and then discharged down to my reserve capacity even after the peak period ended. If the system underestimated my peak usage, fine, then I think it should stop using the battery at peak end and then the system will have less to charge from solar the next day.

I guess to me the ideal setting would have these three goals
1) Avoid Peak Grid usage (Forecast upcoming peak usage and charge battery to reserve + forecast)
2) Minimize total Grid usage (on weekends; understand that next peak period is nearly 3 solar cycles away)
3) Reserve % for Grid Outage
 
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BrettS

Active Member
Mar 28, 2017
2,109
2,512
Orlando, FL
Maybe part of my confusion here is understanding how the Advance (balanced) system is trying to function.
Is there any reason for my battery to have more than the amount of estimated peak usage + my reserve %?

Think of it this way. If your powerwall only has the reserve and the estimated peak usage amount and then for some reason you use more power during your peak time and the powerwall gets down to the reserve then you’ll be paying peak rates for that extra amount.

On the other hand, If your powerwall was allowed to charge to 100% during off peak then your house might use a little more grid power during off peak, but you will be fully covered during the peak time. In this case if you exceed the estimated peak usage amount then the powerwall will have extra power to cover it and you will have effectively only paid off peak rates for that extra peak usage.

If the powerwall charges to 100% and it’s not used during peak, that’s OK. The power in the powerwall won’t disappear, so on the next cycle your powerwall will only need to replenish what was used during peak. So it’s not like it’s adding extra power to the powerwall every time.

Additionally that extra power could be used if you have an outage in the middle of the night, or if you have a rainy day and you’re don’t have enough solar to charge your powerwall enough to make up for the last cycle’s peak usage.
 

miimura

Well-Known Member
Aug 21, 2013
6,165
5,761
Los Altos, CA
Keep in mind that the system knows that the weekend is all Off-Peak if you have it set that way. Many people have observed that it will drain more than usual on Friday evenings because it knows that energy can be replaced with weekend Off-Peak generation. Also, keep in mind that it's better to charge the batteries more full because you don't know when there will be an exceptional consumption day or a sudden drop in solar production.

There is no doubt that the system could be optimized more if it had more pricing information available. Special cases like NBCs, low or no credit for feed-in (export to grid) and the actual prices for each TOU period would be very useful in optimizing the actual cost savings.

As it is, I think it does a pretty good job when I have enough solar production. I'm now on PG&E EV2-A which has Off-Peak generation until 3pm 7 days a week. Except when we have these crazy heat waves and wildfire smoke, it can usually power the house from battery from 3pm-midnight, which is all of the Part-Peak and Peak periods.

There are other people in Arizona that are trying to use the PWs for Demand Charge minimization. There is no Peak Demand on the weekends, so they charge the batteries to full on the weekend and use the PWs to Self-Power the house during Peak hours Mon-Fri. Gradually the batteries are drained lower and lower as the week progresses until they're topped off on the following weekend.
 
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getakey

Active Member
Jan 28, 2020
1,115
359
95762
Think of it this way. If your powerwall only has the reserve and the estimated peak usage amount and then for some reason you use more power during your peak time and the powerwall gets down to the reserve then you’ll be paying peak rates for that extra amount.

On the other hand, If your powerwall was allowed to charge to 100% during off peak then your house might use a little more grid power during off peak, but you will be fully covered during the peak time. In this case if you exceed the estimated peak usage amount then the powerwall will have extra power to cover it and you will have effectively only paid off peak rates for that extra peak usage.

If the powerwall charges to 100% and it’s not used during peak, that’s OK. The power in the powerwall won’t disappear, so on the next cycle your powerwall will only need to replenish what was used during peak. So it’s not like it’s adding extra power to the powerwall every time.

Additionally that extra power could be used if you have an outage in the middle of the night, or if you have a rainy day and you’re don’t have enough solar to charge your powerwall enough to make up for the last cycle’s peak usage.

I totally agree. Using "forecast peak + reserve" as the projected amount to charge can result in using peak power from grid if forecast is wrong. Also, there's always that unexpected outage that could exceed your reserve. Having more charge is rarely a liability if at all

NBCs are what 2 to 3 cents/kWh? Not enough for me to worry about

Underestimate peak usage by 1kWh, you lost $.50 compared to trying to save $.03
 
Last edited:

MJ_CA_2019

Member
Aug 19, 2020
107
26
Central CA
I totally agree. Using "forecast peak + reserve" as the projected amount to charge can result in using peak power from grid if forecast is wrong. Also, there's always that unexpected outage that could exceed your reserve. Having more charge is rarely a liability if at all

NBCs are what 2 to 3 cents/kWh? Not enough for me to worry about

Underestimate peak usage by 1kWh, you lost $.50 compared to trying to save $.03

That's a helpful perspective and yes I agree, it would be bad to underestimate and pay peak rates.

I'm trying "Cost Savings" today (Sunday) as I noticed that on "Balanced" the system was discharging my battery while the sun is down.
Battery charged to 100% then started discharging. Again that seems to me to be causing unnecessary non-bypassable charges.
 

jjrandorin

Moderator, Model 3, Tesla Energy Forums
Nov 28, 2018
8,147
9,003
Riverside Co. CA
That's a helpful perspective and yes I agree, it would be bad to underestimate and pay peak rates.

I'm trying "Cost Savings" today (Sunday) as I noticed that on "Balanced" the system was discharging my battery while the sun is down.
Battery charged to 100% then started discharging. Again that seems to me to be causing unnecessary non-bypassable charges.

I believe those changes to advanced modes take a few days for the algorithms to settle in so changing them daily may not behave as you expect. At least, I read that here somewhere, I dont use them, I use self powered

Also, how is discharging your battery into your home generating NBCs? discharging the battery into the home can run your home with your main breaker in the off position, so how is that generating NBCs?
 

MJ_CA_2019

Member
Aug 19, 2020
107
26
Central CA
Have you tried Self-powered with 100% reserve? This is not exactly the same as Backup mode.

This would work if I wasn't on a TOU plan. I want the system to avoid Peak rate charges and I have the PW capacity to do this even with a 25% reserve for outages. It appears that I will have to settle for the Advanced setting of "Cost Savings". I've had my PW in this mode all of this week and the behavior is pretty close to what I want.

On Monday I exported 6 kWh to the grid in Off Peak time. That is because 100% of my Solar was going to charging the battery earlier in the day. As I mentioned previously I think it would be possible to configure the behavior such that the system was charging the battery whenever excess solar existed and draw from the grid when solar production is exceeded. Instead, the system takes 100% of the home load from the grid and puts 100% of the solar production to the battery. Current behavior causes unnecessary non-bypassable charges.
 
Last edited:

getakey

Active Member
Jan 28, 2020
1,115
359
95762
This would work if I wasn't on a TOU plan. I want the system to avoid Peak rate charges and I have the PW capacity to do this even with a 25% reserve for outages. It appears that I will have to settle for the Advanced setting of "Cost Savings". I've had my PW in this mode all of this week and the behavior is pretty close to what I want.

On Monday I exported 6 kWh to the grid in Off Peak time. That is because 100% of my Solar was going to charging the battery earlier in the day. As I mentioned previously I think it would be possible to configure the behavior such that the system was charging the battery whenever excess solar existed and draw from the grid when solar production is exceeded. Instead, the system takes 100% of the home load from the grid and puts 100% of the solar production to the battery. Current behavior causes unnecessary non-bypassable charges.

What TOU plan are you on?
 

MJ_CA_2019

Member
Aug 19, 2020
107
26
Central CA
I've been on the Cost Savings with 25% reserve setting for over a week now:

Saturday - Sunday
100% of solar goes to PW until Battery 100% then PW in standby mode

Monday starts the day at 100% battery charge so the PW was in standby until 4:00 pm

Tuesday - Friday behavior
PW only discharges during Peak times (M-F 4-9 pm)
Battery % at the end of peak time was between 45-55% each weekday.
100% of solar goes to charge PW until Battery at 100% regardless of day of the week.


Here are some additional observations:
Battery reaches 100% between 12:45 - 1:45 pm (Tue - Fri) and exports excess to the grid until 4:00 pm
Solar production first exceeds home loads between 10:00 - 11:00 am each day (I have my pool pump timer set to kick on around 9:00 am)
Peak Period Home Usage averaged: 19.1 kWh
Peak Period Solar Generation averaged: 7.2 kWh
Off Peak Solar Export average: 4.9 kWh


Do you think the Tesla algorithms would shift behavior to charging from excess instead of sending 100% of solar to the battery if I shifted my pool pump time (i.e. lowered my home usage in off peak while Solar active) ?
I think what is bothering me here is that the charging behavior described is the same between the two modes, but I'm not seeing that behavior.

I may try a week on Balanced mode but what I expect to be different is only that the PW will discharge down to my reserve % each day.



upload_2020-9-5_11-51-22.png
 

getakey

Active Member
Jan 28, 2020
1,115
359
95762
I still don't get what you are trying to achieve better than what it is doing under Cost Saving mode. You want 100% solar going to PW to get it 100% charged so that PW can power house completely through the Peak. This also maximizes the solar going back to grid during peak.
 

MJ_CA_2019

Member
Aug 19, 2020
107
26
Central CA
I still don't get what you are trying to achieve better than what it is doing under Cost Saving mode. You want 100% solar going to PW to get it 100% charged so that PW can power house completely through the Peak. This also maximizes the solar going back to grid during peak.

Two things are currently on my mind as I currently use Cost Savings

1) The PW charging behavior is not as advertised (See post 18) because the charging behavior is not excess solar, it is 100% of solar. Since I am exporting to the grid during off peak (battery reaches 100% hours before the peak period starts), this means unnecessary grid usage during off peak (and non-bypassable charges).

2) When we get to the time of year where my solar production consistently exceeds my home usage I question whether the cost savings mode will recognize that it is capable of effectively going off grid. If it doesn't, then again I'll be incurring unnecessary non-bypassable charges.
 

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