Powerwall 2: TOU/Rate Arbitrage

[abasile]

Powerwall 2 battery usage customization update out today for iOS! I believe I am at 59% or so when the solar starts to kick in to refill the battery in the morning so I set it at 50% reserved for backup power for now. Will adjust as the time of year and weather changes. Very nice to see this capability added today!

Great to see this progress! Next, Tesla needs to allow PW2 owners to customize the time period during which to draw power from the battery. While we don't generate enough solar energy to eliminate grid usage, we want to be self sufficient during "peak" hours, which are 2pm - 8pm on our TOU rate plan. Because of shading from tall pines, our solar production really drops off after 2pm, and the PW2 should fill this gap nicely. However, after 8pm, it would be counterproductive to draw from the PW2 unless there's an outage.

 

[stevemcsheen]

Agree 100% on the TOU adjustments. They would make a big difference for you. For me, I would want to draw from the PW2 after my panels stop producing as much as my home consumes, around 7pm these days. (this would vary by season and weather) My peak period is 2-9pm with PG&E's EV TOU plan and I would rather use the battery from 7-11pm. 11-7am off-peak is less of a concern, but why not be totally off the grid in the summer if you can be? Also, my yearly consumption is more than my production so I'm not getting paid for what I send back to the grid so why not use it overnight?

As it is, looking at the app after the update, it's 9:40pm and it hasn't switched to battery even though I set it to do so after dark. So either something's wrong or it takes 1 daily cycle to kick in.

 

[Jeffgtx]

Actually that was the root of my question. It seemed to me that setting at 50% was going way over the discharge requirement since technically you only need to do 1/7th each night to meet it. So I was curious if you were being overly conservative on the setting and were going to sacrifice some backup hours.

I'm only asking because I haven't figured out what strategy I want to employ so I was looking for insight from you.

Thanks.

 

[stevemcsheen]

Got it. When I tried turning off the power to the home completely in the evening when the house demanded more than solar was producing and then turned it back on in the morning when the solar was producing enough again, I'd be at around 59%. (assuming I didn't have to charge the car completely overnight) So I set it at 50% to at least be under that with some room to spare just as a v1.0 shot at it.

Battery didn't kick on overnight at all, so something's up. I guess I'll see tonight if a full 1 day cycle is required for it to kick in or if the message isn't getting to the gateway to switch to the battery at night . . .

 

[2012MS85]

Battery didn't kick on overnight at all, so something's up. I guess I'll see tonight if a full 1 day cycle is required for it to kick in or if the message isn't getting to the gateway to switch to the battery at night . . .

stevemcsheen, I'm very curious if you were able to get your PW2 to kick on and discharge? I'm hoping so because my plan for my PW2 is to fill it up with my solar production from 11am-4pm (since I'm producing much more solar energy than my daytime needs when no one is home), but then discharge the battery's entire 13.2 kWh of energy during my utility's peak TOU from 4-9pm. I'd like to be able to use the iOS app to "set" a flat discharge rate of 2.64 kWh during each of the 5 peak hours where my utility charges 16 cents per kWh (double Iowa's off-peak rate of 8 cent). If we don't need to run our A/C, we only use about 1 kWh every peak hour, so I'd like the extra battery discharge to go back to the grid (where my utility actually pays a monthly cash out at 11 cents for customer-generated peak energy from 4-9pm, which is far better than my monthly net metering cash-out of my excess off-peak energy at 3 cents. Hopefully Tesla gives us this level of autonomy to control the PW2 as desired ;o)

 

[2012MS85]

MS85, I just got a call from Powerwall support about my email to them. It was a little difficult to parse the VM they left - I couldn't take the call - but it sounds like either new firmware is being pushed automatically to the gateway over the next 24-48 hours or there's an app update coming in the next 24-48 that will push this update to the gateway on its own. They're going to call me back on Thursday to follow up. I'll try to update you then. Unless someone else has had a similar call or experience?

The PW2 does indeed power the house if I manually shut off main home power, but this app update and the PW2 page mention being able to automatically charge from solar during the day and use it at night. There's no finer grain control over it than that, though - at least not yet.

Thanks for that update; please let us know how today's follow-up goes. I would not want to have to manually shut off my main home power in order for the PW2 to work as desired, so I'll be glad to hear that Tesla is on top of this and designing the app, hopefully, to accomplish this via a software update on Tesla's app. Congrats on your PW2 install; lots of us wish we had ours already ;o)

 

[montreid]

Anyone with PW2 installed fully discharge their PW2 during peak TOU and then charge overnight in super off peak rates for full on arbitrage? We're Netmetered on EV-TOU -- the PV is undersized for total house consumption with two EVs. During daytime we're ahead, but when kids get home we start outpacing the PV at 5pm and draw during shoulder times until 9pm.

Ideally would want to be neutral or ahead during all periods of peak and able to discharge the battery during any of that time, and then set the charge from 12midnight to 5am on the battery to be ready for the next day. Does the PW2 program allow for that?

 

[zedkyuu]

Anyone with PW2 installed fully discharge their PW2 during peak TOU and then charge overnight in super off peak rates for full on arbitrage? We're Netmetered on EV-TOU -- the PV is undersized for total house consumption with two EVs. During daytime we're ahead, but when kids get home we start outpacing the PV at 5pm and draw during shoulder times until 9pm.

Ideally would want to be neutral or ahead during all periods of peak and able to discharge the battery during any of that time, and then set the charge from 12midnight to 5am on the battery to be ready for the next day. Does the PW2 program allow for that?

You can do it with the app right now, but it's clunky. The app lets you set two modes: backup only and self-powered. Backup only charges you at max rate from anywhere to 100% and then keeps it there unless the power goes out. Self-powered charges you at max rate from anywhere to your specified reserve battery level, then charges from there only off PV. It discharges at up to max rate off the battery until you hit the reserve battery level, and then stops discharging and runs your house off the grid.

So if you did something where you set backup only mode at night, then set self-powered mode with a 0% reserve level when the morning comes around, you could approximate what you want. It wouldn't match it exactly, but with some tweaking, you could get pretty close. But it's tedious.

Considering that PW2 installers insist on having it connected via Ethernet, I keep wondering if we'll get any ability to control it that way. A program running on a server that manages the PW2 would be pretty nifty.

 

[montreid]

Thanks zed; so in self-powered mode with 0%; it would discharge at a steady rate regardless of household load or only whatever load the house demands? How about when solar generation is made?

So this is what I'd like to do and you tell me if it can be done currently:

1200midnight: Battery 0% - full discharged -- Backup set on and battery charges to 100% in 1-5 hours at super -off peak
5AM: Battery 100% B Backup setting to prevent discharge
6AM-10AM: PV kicks in but not 100% balance - since off peak - grid will support home -- PW2 set as back up still
10A-5P: PV generation excess goes to grid Peak hours - PW2 100% --- ?: can you FORCE PW2 discharge at this time to say 50%?
5P-12MN: PV generation drops; switch PW2 program to self-powered mode and support throughout the evening-night --- can you FORCE PW2 to discharge completely at shoulder rate?
12MN - Battery 0% and ready to cycle again.

 

[zedkyuu]

The battery discharges only as much as household load or max battery rate (5 kW per battery), whichever is less. I don't see anything inside the app that lets you configure the battery to discharge to the grid. It must be possible, but I imagine Tesla isn't pushing the pure arbitrage angle as much.

I should clarify that I've never charged the battery to full on battery only. I typically run in self-powered mode at 40% reserve capacity, and at that point, solar charges the battery to a maximum of 90%. However, I can configure a reserve level of 100% in self-powered mode, and I don't know what happens when I do that. Haven't run on battery only long enough to charge it to that point, either.

Anyway, when solar generation is available, the behaviour depends on whether the battery charge is above or below the reserve level. If it's below the reserve level, the software tries to charge the battery as quickly as possible, so it'll charge it from both available solar and the grid. If it's above, then it'll only charge from solar. Factoring household loads into the mix, we get these scenarios:

Battery above reserve level, solar generation greater than house loads:
- Battery is charged using leftover solar generation
- If battery is fully charged (90% in my case, see note above), leftover solar generation goes to the grid

Battery above reserve level, solar generation less than house loads:
- Battery discharges to cover the difference between solar and house load
- No power to/from grid

Battery below reserve level:
- Battery charged at maximum rate using grid and any remaining solar generation after house load is met
- If house load exceeds solar generation, then battery is charged entirely off the grid and grid supplies the differential house need

Mapping this to your scenario, I get:

Midnight (battery 0%):
- Set battery to backup-only mode
- Battery charges to 100% off grid at max rate (about 1 h 20 min per battery)
- House loads supplied off of grid

6am-10am:
- PV kicks in
- Grid still off-peak
- Leave battery in backup-only mode; house loads supplied off of PV and grid

10am-5pm:
- Grid goes to peak
- Switch battery to self-powered, reserve level 0%
- House loads supplied off of PV and battery (unless battery maximum of 5 kW per battery is hit, in which case, grid supplies the rest)
- Any excess PV first recharges battery (to 90%; see above), and then goes to grid
- If battery is discharged at any time during this period, house loads revert to PV and grid

5pm-midnight:
- PV down
- House loads supplied entirely off of PV and battery, just like 10am-5pm
- Once PV falls off entirely, house supplied off of battery to battery max of 5 kW/battery, and then grid supplies the rest
- Once battery hits 0%, house supplied by grid entirely

Regarding your question on whether it's possible to force the battery to discharge even when sufficient PV is available to charge it, I don't see any way to do this, unfortunately. This would be similar to doing arbitrage by charging the battery at off-peak and then forcing the battery to discharge into the grid at peak, and there's no way to force the battery to discharge that way. Nor is there a way to force the battery to not charge even when there is available PV. These seem like arbitrary limitations removable in software, but I could see Tesla being less interested in giving its customers the means to profit maximally off this arbitrage.

 

[miimura]

I would like to set a minimum discharge rate during Peak hours of 2pm-9pm of approximately 1kW per PowerWall, regardless of house load. Obviously, if the house is drawing more than that, it should match the demand.

Actually, a better strategy would be to tally the daily solar charging and have a smart algorithm that will scale the Peak period discharge so that the entire amount of solar that went into the battery would go out during the Peak period even if the house is not using it. If I set a reserve of 25% and it only charges to 90%, then there should be 9kWh/day per PowerWall to play with.

 

[Skryll]

Thanks @miimura again. I found error my spreadsheet, in the EV-A calculation I was referring to an empty cell for partial peak during the summer period. It became pretty obvious the error was going to be be there when I did side by side comparison with your spreadsheet. Now they give the same guidance, EV-A is much better than E-6.

arnold

Is there any difference in time-guarantee for E6 vs. EV-A, i.e. right now EV-A seems much more favorable because of the higher rate at peak and lower rate at off-peak, but how long are those rates locked in ?

I know that if I switch out of E6 to EV-A that I can't switch back.

 

[miimura]

Is there any difference in time-guarantee for E6 vs. EV-A, i.e. right now EV-A seems much more favorable because of the higher rate at peak and lower rate at off-peak, but how long are those rates locked in ?

I know that if I switch out of E6 to EV-A that I can't switch back.

I'm not sure I understand your question about "time-guarantee". The time schedules in the definition of each rate schedule generally don't change (ie. Peak is always 2-9pm) while the $/kWh changes very often. I believe that the inability to change the hours in the schedule is why they have to retire certain rates like E-7. It just didn't fit the grid economics any more.

Anyway, if you have an EV and can routinely charge it between 11pm and 7am, I am pretty sure you will be better off with the EV rate schedule than E-6. The difference in how much more solar dollars you can capture on E-6 is more than offset buy how much more it costs you to charge your car. At least that's what I have seen in the calculations based on my usage patterns.

 

[arnolddeleon]

I'm not sure I understand your question about "time-guarantee". The time schedules in the definition of each rate schedule generally don't change (ie. Peak is always 2-9pm) while the $/kWh changes very often. I believe that the inability to change the hours in the schedule is why they have to retire certain rates like E-7. It just didn't fit the grid economics any more.

Anyway, if you have an EV and can routinely charge it between 11pm and 7am, I am pretty sure you will be better off with the EV rate schedule than E-6. The difference in how much more solar dollars you can capture on E-6 is more than offset buy how much more it costs you to charge your car. At least that's what I have seen in the calculations based on my usage patterns.

I'm guessing he was asking how long EV-A is going to be available. E-6 has a known sunset date, could EV-A disappear before E-6 goes away? If that is the case he wouldn't be able to go back to E-6.

arnold

 

[Skryll]

I'm guessing he was asking how long EV-A is going to be available. E-6 has a known sunset date, could EV-A disappear before E-6 goes away? If that is the case he wouldn't be able to go back to E-6.

arnold

No, I was assuming that the rates on E6 are somewhat fixed and don't change over time. But I don't know that, sounds like that was wrong. The reason why i did not want to switch out of E6 is because I can't go back and he TOU hours are better for solar and electric car charging (peak starts earlier and partial peak ends earlier) but the actual cost per kWh of EV-A is making EV-A better than E6 regardlessly. But for how long, what if in a year that all gets changed and then I am stuck with a worse time schedule and reset rates that make the end result worse than having staid with E6.

 

[miimura]

Last night when I did "customize" to shift solar to evening with backup 25% reserve, it used up anything left last night until it got down to 25%, then I set it to 20% and it used it down to 20%. Now it's using only solar to charge itself. This makes perfect sense.

Before I had the batteries installed, I was earning Day rate credits during peak solar output. Now I'm not earning those valuable day rate NEM credits. As the solar output fills the battery, it will start using solar for the grid. I can tune the backup reserve to take in less solar so that I can get more solar NEM credits. I'll have to see how this balances out.

When I calculated the arbitrage potential using the PG&E EV-A rate schedule, I assumed that the battery would be charged 100% from solar and you would be giving up those Part-Peak NEM credits and it would be discharged during Peak period. Presumably during the Summer season the solar would completely re-fill the battery before the peak period started. Any generation during Peak would receive full NEM credit and when the sun goes down and the household usage exceeds the solar generation, the Powerwall would supply the house. The net effect is that you are foregoing Part-Peak credit in exchange for zero Peak consumption while retaining your Peak credits.

I calculated $700 per PowerWall per year of arbitrage potential, but it assumes 10kWh/day discharge during Peak and I don't think you can force the current software to discharge into the grid beyond your consumption. So, in order to calculate your Arbitrage potential you would have to know your actual consumption during the Peak period.

 

[Ulmo]

When I calculated the arbitrage potential using the PG&E EV-A rate schedule, I assumed that the battery would be charged 100% from solar and you would be giving up those Part-Peak NEM credits and it would be discharged during Peak period. Presumably during the Summer season the solar would completely re-fill the battery before the peak period started. Any generation during Peak would receive full NEM credit and when the sun goes down and the household usage exceeds the solar generation, the Powerwall would supply the house. The net effect is that you are foregoing Part-Peak credit in exchange for zero Peak consumption while retaining your Peak credits.

I calculated $700 per PowerWall per year of arbitrage potential, but it assumes 10kWh/day discharge during Peak and I don't think you can force the current software to discharge into the grid beyond your consumption. So, in order to calculate your Arbitrage potential you would have to know your actual consumption during the Peak period.

I agree.

Today is 0 day.

It continues:

 

[abasile]

I wonder if the app now allows you to configure a time in the evening past which to use the grid rather than discharging from the PW2.

Specifically, with SCE, our TOU peak rates apply from 2pm - 8pm. I'd like to use our awaited PW2 to eliminate all grid usage during those hours. However, after 8pm, it'd be financially counterproductive to discharge the PW2 any further. (Yes, I understand there are environmental reasons to eliminate all grid usage, but with our two EVs and household usage, we don't have enough solar production to accomplish that anyway.) After 10pm, "super off peak" rates apply and discharging the PW2 would make even less sense, dollar-wise.

 

[Ulmo]

I'm discharging heavily, because it's normal weather here in Aptos, i.e., cold (right now a very typical and normal 56º), and of course I have a heater on. I use an electric heater. A few mountain ranges over it's in the 70s, and during the day in the 90s. I don't know what it was here earlier without looking online since I was one mountain range over all day for work, and out there, it's warmer, too.

I was pleasantly surprised to see how much is left even with this rabid use:

I could "Customize" it daily to raise the backup reserve to cut off the discharge of the PowerWall at night when rates are good and lower the backup reserve before the evening heavy use and heavy rate period, but doing that there's not as much clean energy from PG&E as there is from the home's roof. I think I'll go pure for today and see what happens.

I read here that others are having trouble with the panel upgrade thing. I already did a panel upgrade prior to the work being done back when I had my solar installed, since I knew it was needed. I already ate that cost prior to Tesla's involvement. Essentially, one cheap way to do a panel upgrade to break it up into two panels if your existing panel is good or you're assumed to have upgraded it separately is to keep it as is but convert it to a subpanel, and just stick up a metered panel as the main service panel. But, I suppose that would be considered this "panel upgrade" cost you are talking about. Essentially, I'd add that to the list of common problems identified that need to be made easier. Perhaps Tesla's switch box can double as a main panel and meter; they'd do well to turn that box into a higher number of purposes; then, the "panel upgrade" is just Tesla sticking up the box they were going to stick up anyway, and then ... and then I guess that's it.

I still have to spend part of Sunday taking apart their new installations to see what they did. For instance, how does my subpanel get fed? Is it given a full 200 amps, or did they drop it to 100 amps?

 

[Dave EV]

I wonder if the app now allows you to configure a time in the evening past which to use the grid rather than discharging from the PW2.

Specifically, with SCE, our TOU peak rates apply from 2pm - 8pm. I'd like to use our awaited PW2 to eliminate all grid usage during those hours. However, after 8pm, it'd be financially counterproductive to discharge the PW2 any further.

Yep, to maximize financial ROI, I'd want to be able to program in "peak" grid hours where you stop charging the Powerwall from solar and allow the Powerwall to discharge to avoid importing grid power during peak hours.

With a 3 TOU periods (super-off-peak, off-peak, on-peak) and NEM solar, there's really no incentive to self-consume except during on-peak periods. And with super-off-peak periods typically when it's still dark, there's no incentive to discharge then or during off-peak, either, especially when you consider that you only get 89% of the energy out of the Powerall that you put into it. Self consumption basically just costs you money otherwise, unless you're under NEM 2.0 and have non-bypassable charges, but even then the non-bypassable charges are low enough that at best it's a wash, so it doesn't really matter what you do.

But I suspect that we'll see shifting of peak periods later into the evenings and perhaps even a short peak period in the early morning as more and more solar gets on to the grid.