Strategies for Powerwalls and Utility Demand Plans

[DanAman]

My utility puts new solar customers on a demand plan called the "Solar Price Plan".

The Solar Price Plan is the price plan for residential customers who choose to produce some of their own energy using rooftop solar. The plan encourages customers to shift their energy usage away from peak times. Customers who are able to avoid large demands during peak times can save significant money with a much lower energy rate during the rest of the day.

How Does It Work?
The Solar Price Plan is a combination of two parts:
1.) A demand charge applied during a peak period
2.) A lower energy rate than the standard plan

A demand charge is applied based on your single highest energy usage during peak times over a billing period. The demand charge is $5.27 per kilowatt. For example, if your highest usage during a peak period for a month is 5 kilowatts, your demand charge would be $26.35.

All your energy for the month is billed at $0.2427 per kilowatt hour (kWh) regardless of when it is used. This rate is less than half what you would pay on the standard plan.​

It's a carrot and stick plan. They allow net metering in exchange for placing you on the plan. The plan gives you a lower per kWh price, but you pay for demand. This was the reason I installed powerwalls. In central Florida, it rains almost every day in the summer, and AC is a requirement for most people. In the winter, we produce almost 40% less energy. All it takes is one cloud at the wrong time to spike your bill $20 to $50. This plan nearly negates the benefits net metering, unless you add storage.

In the past I actively managed my powerwall to make sure I had enough energy stored to make it through the demand period while enjoying high demand devices like the HVAC, stove, and oven. One to two hours before the demand period began I would set the powerwall to self-powered mode with a low reserve. Then once the period passed, I would set the powerwall back to backup only with a reserve of 100%. I learned the hard way that the powerwall can take a long time to switch modes. Now I keep the powerwall in self-consumption mode and adjust the reserve.

I tried using Advanced Time Based Controls, but found that the behavior to be strange and unpredictable. The powerwalls would randomly send power to the grid when I would have preferred the batteries to be charged. Other times it would pull power from the grid even though the batteries were full. All of this happened during the defined peak periods. Then when in off peak, the powerwalls would continue to discharge until empty. After many calls to Tesla and their explanations not making sense, along with the documentation not matching up with actual behavior I gave up on Advanced Time Based controls.

I have gone back to changing the self-consumption reserve before and after the demand period. I'm wondering how others on demand plans manage their powerwalls, if at all. As a side note, we originally has a western facing solar array. I came to realize that the rain patterns in this area meant that we received less sunlight because it rained in the afternoon. We installed and additional eastern facing array and are now seeing how that affects our supply, consumption, and management of the powerwall.

 

[DanAman]

Here's something I learned along the way. Our Tesla Energy Gateway (TEG) is connected via ethernet to a switch. If I

1. change the reserve in the app
2. power down the port or unplug the ethernet cable
3. wait 30 to 60 seconds
4. power up the port or plug the cable back in

The TEG syncs with Tesla and pulls in the updated settings. I'm guessing disconnecting the cable forces it to fallback cellular and it checks for new settings while making the switch. I use this if I want to force a quick change. It does not work as well for going back to backup mode. Changing out of Time Based Control mode seems to take longer, up to 24 hours. Resetting the TEG is the best way to force that change.

 

[Darwin]

My PowerWall was installed a few months ago, and so far it has helped manage my utility's demand charge well. I'm currently using Time-Based Control and with a single Shoulder period (no Peak) to cover my utility peak hours. After two months of PowerWall operation, my maximum demand during peak periods has been 200W even with a 4kW air conditioner running periodically (day and night). It's all been relatively hands-off for me.

I am using a Shoulder period insteak of a Peak period since based on my interpretation of Tesla's modes of operation with solar, in Peak periods, excess energy can be sent back to the grid instead of being used to charge the battery - which doesn't help demand one bit. In Shoulder periods, from my experience, the battery will always be charged if there is excess solar - which is always what I want.

I'm in the Phoenix area and my utility is SRP.

 

[DanAman]

Can you post a screenshot of your settings? Are you using balanced or cost savings? I think I tried this before, but I am willing to try again. I can experiment on the weekends where failure does not cost anything. Also, how do you interpret the charge and discharge priority or advanced time based controls? It's pretty easy for self-powered. For me cost savings looks like a no go because it will charge with any solar, and discharge with any usage in shoulder. I need it to be excess solar for charging, and net usage for discharging. It would seem like balanced could work, but charge priority 3 and 4 are potential problems.

Right now my demand in self-powered mode as measured by the meter is 0.033kW. It was a lot higher when I experimented with time based control. I checked out your demand plan and it looks a lot more brutal than my utilities plan. I have a single peak in the winter and the fixed charges are about 1/3. The utility did implement a dual peak in Oct 2018 but went back to single after complaints from solar customers.

 

[Darwin]

Yes, SRP's demand plan is brutal (and punitive and draconian) for solar customers, but the PowerWall is helping immensely so far. I'm using Cost-Savings, but frankly I haven't noticed any real difference between Cost-Savings and Balanced when in shoulder periods.

Here are screenshots of my settings showing a single Shoulder period from 5am-9pm (by defining the off-peak period from 9pm-5am):

As you probably noticed, SRP has both a winter morning peak period (5am-9am) and an evening peak period (5pm-9pm). I am using a single (5am - 9pm) Shoulder period to cover both. I really doesn't matter if 9am-5pm is off-peak for my utility (but treated as shoulder by the PowerWall) since my solar kicks in around 8am anyway and covers all net usage and fully charges the battery regardless. My PV system is 5.6kW.

My reserve is cranked up to 40% just to keep the battery from using extra cycles to discharge excessively in off-peak. I have no idea why it discharges at all in Cost-Savings Off-Peak periods based on Tesla's posted PowerWall rules. Aside from the excess battery cycles, it doesn't appear to hurt anything, although it does add some risk to demand management if the battery discharges too optimistically and the following day is cloudy.

I find the Tesla Operation Summary table quite confusing, particularly for Cost Savings:

Cost Savings-Charging Priority: 1. Any solar (Off-peak) 2. Any solar (Shoulder)
Cost Savings-Discharging Priority: 1. Any usage (Peak) 2. Any usage (Shoulder)

I look at the charging and discharging as kind of a net cloud effect. Even though it says the battery will discharge for any usage in Shoulder, it appears that since the battery will also charge with any solar in Shoulder, the net effect (at least for me) is that the battery will really only discharge for net usage in Shoulder.

From my experience, it appears to me that in Shoulder periods, the net result is that solar will first be used for net usage, then sent the battery, and any remainder will be sent to the grid. Discharging only occurs in Shoulder if solar cannot cover net usage.

I may be more than a little off with some of this, but it does appear to be working well for the last couple of months. Just perusing the forums though, it appears that experiences can vary wildly, so I'm wondering if there are more factors at play...

 

[power.saver]

Can you post a screenshot of a typical day's performance showing the charge/discharge with this schedule?

 

[Darwin]

Can you post a screenshot of a typical day's performance showing the charge/discharge with this schedule?

Here a screenshots from today for the 5am-9pm shoulder period in cost saving mode. There's quite a bit of clutter, so I separated out the From Grid and Home Usage plots. The only noticeable grid usage that occurred during the shoulder period was a small spike around 5pm, which will probably result in a 200W or less peak demand for the day.

The ~4.5 kW usage spikes in the afternoon/evening are due to the air conditoner and the 9kW usage starting around 10pm is due to EV charging.

You can see the battery started discharging around 1am during off-peak - about 4 hours before the shoulder period started. This is pretty typical unless I keep the reserver higher. Aside from being annoying and wasteful from a demand management/battery cycle perspective, the early battery discharge doesn't hurt the demand management goals unless it ends up being a very cloudy day.

 

[JES2]

Here's something I learned along the way. Our Tesla Energy Gateway (TEG) is connected via ethernet to a switch. If I

1. change the reserve in the app
2. power down the port or unplug the ethernet cable
3. wait 30 to 60 seconds
4. power up the port or plug the cable back in

The TEG syncs with Tesla and pulls in the updated settings. I'm guessing disconnecting the cable forces it to fallback cellular and it checks for new settings while making the switch. I use this if I want to force a quick change. It does not work as well for going back to backup mode. Changing out of Time Based Control mode seems to take longer, up to 24 hours. Resetting the TEG is the best way to force that change.

Excellent. I will try this tomorrow as my peak begins. I'm sure the Tesla Energy folks will love this approach.

 

[power.saver]

Here a screenshots from today for the 5am-9pm shoulder period in cost saving mode. There's quite a bit of clutter, so I separated out the From Grid and Home Usage plots. The only noticeable grid usage that occurred during the shoulder period was a small spike around 5pm, which will probably result in a 200W or less peak demand for the day.

The ~4.5 kW usage spikes in the afternoon/evening are due to the air conditoner and the 9kW usage starting around 10pm is due to EV charging.

You can see the battery started discharging around 1am during off-peak - about 4 hours before the shoulder period started. This is pretty typical unless I keep the reserver higher. Aside from being annoying and wasteful from a demand management/battery cycle perspective, the early battery discharge doesn't hurt the demand management goals unless it ends up being a very cloudy day.

View attachment 402814 View attachment 402816 View attachment 402815

That is interesting, but I'm not liking the off-peak discharge from 1am on. It doesn't make sense. I use only off-peak and peak (no shoulder) to ensure discharge only during the peak, and your off-peak and shoulder (no peak) intrigued me. I was hoping to see no discharge off-peak and a more balanced (self consumption type) charging/discharge during the shoulder.

Thanks for posting the screenshots.

 

[Darwin]

I'm not liking the off-peak discharge from 1am on.

I don't like it either. I can generally keep it from happening by appropriately setting the reserve each night, but I stopped putting in the effort after awhile as it was otherwise managing my demand during peak almost perfectly. I'm looking for a hands-off approach for demand management, and this is the best I've found so far for my needs.

For my SRP utility demand plan, energy is pretty cheap ($0.036- $0.06 per kwH) and there's little difference between on and off peak price, so it doesn't matter much to me that the battery discharges off-peak from that perspective, but it's a completely wasted battery cycle both from a degredation and efficiency perspective. Is it a bug or does Tesla really want you to cycle that battery up to its reserve every day regardless?

What matters for my plan is that I consistently have zero or negative net usage during peak hours.

I've contemplated playing around with adding a short peak period in there somewhere to see if that would stop the off-peak discharge, but the last thing I want to do is have it prioritize the peak period over the shoulder to the point that it ends up drawing from the grid during my utility's peak. Similar to @DanAman's situation, a single half-hour mistake at 5kW can cost me a demand charge upwards of $50.

Just as a side note, on the current plan, if I accidentally charge the car just once in off-peak for 30 minutes at 10kW, it could result in a $200 demand hit in the summer. There's not much a single PowerWall can do though in that situation anyway, so you really need to stay vigilant regardles. SRP started offering a demand assurance program that provides a form of gradually diminishing forgiveness after Tesla sued them back in March 2018, so that helps a little. At least for the first time you blow your demand.

 

[power.saver]

Just as a side note, on the current plan, if I accidentally charge the car just once in off-peak for 30 minutes at 12+kW, it could result in a $200-$400 demand hit in the summer.

Did you mean "on-peak" for 30 minutes?

Not having your demand charge rate, I don't know the details of how it affects billing, but with my schedule of only off-peak and peak, I never have any draw from the grid, and there is never any discharge during off-peak either. During peak, it's all discharge and it never draws from the grid unless the reserve is hit, which hasn't happened yet (and I have a 40% reserve).

What I would like is to discharge only the net usage during the peak instead of all usage, and thought your shoulder could do that without the off-peak discharge.

 

[Darwin]

Did you mean "on-peak" for 30 minutes?

Not having your demand charge rate, I don't know the details of how it affects billing, but with my schedule of only off-peak and peak, I never have any draw from the grid, and there is never any discharge during off-peak either. During peak, it's all discharge and it never draws from the grid unless the reserve is hit, which hasn't happened yet (and I have a 40% reserve).

Yes - that should have been "on-peak". It seems there's a catch with both peak and shoulder that enforces more discharge than many of us would like. Initially I was worried that peak might discharge fully before the end of my utility's peak period leaving me hanging with demand. Shoulder seemed more appropriate for demand management for that reason, but maybe I'll get brave enough to try peak at some point if Tesla doesn't address the discharge during off-peak concern soon. I wouldn't worry as much about experimenting if only the PowerWall responded more quickly to setting changes without jumping through hoops.

 

[power.saver]

You are correct that the peak could run you down to the reserve and then you'd be drawing from the grid. I think if you know your typical load, you could figure out if this is likely to happen or not. If it is, then peak probably isn't going to work for you. How many PWs do you have?

 

[Darwin]

My big concern is late summer when I'll have a typical house load of probably about 3 kW if you average between the air conditioner on and off cycles. I have a single PW.

In late summer, I'll need to make it probably 3 hours in the evening (after solar stops producing) with no net grid usage. If I need a 3kW draw from the grid for even a single half hour period during peak utility hours, my demand is blown for the entire month. It looks like a shoulder period will work fine; I'll go into that 3 hour window with a full SOC and make it through easily, but I'm a little shaky on what using a peak period instead would do.

I admittedly haven't looked over many of the graphs posted, but from what I've seen, in a peak period, it looks like the PW may begin discharging even when solar is producing enough to cover home usage. If this is the case, the PW may not start the 3 hour window with a full SOC which sounds too risky for me.

 

[power.saver]

I will start to discharge and cover full house load in the peak, so maybe make the peak just the last 3 hours when you have no solar production. Try it on a day when you know you won't exceed your capacity to be sure though.

 

[eml2]

I was looking at Tesla's Powerwall Support page and notice this under Time-Based Control / Energy Forecast:

Behavior
Powerwall will charge from excess solar during off-peak and shoulder (if required).
Powerwall will discharge during all periods, minimizing exports during shoulder and off-peak.

Powerwall Time-Based Control | Tesla Support

I don't recall seeing this before.

 

[wraithnot]

My utility puts new solar customers on a demand plan called the "Solar Price Plan".

The Solar Price Plan is the price plan for residential customers who choose to produce some of their own energy using rooftop solar. The plan encourages customers to shift their energy usage away from peak times. Customers who are able to avoid large demands during peak times can save significant money with a much lower energy rate during the rest of the day.

How Does It Work?
The Solar Price Plan is a combination of two parts:
1.) A demand charge applied during a peak period
2.) A lower energy rate than the standard plan

A demand charge is applied based on your single highest energy usage during peak times over a billing period. The demand charge is $5.27 per kilowatt. For example, if your highest usage during a peak period for a month is 5 kilowatts, your demand charge would be $26.35.

All your energy for the month is billed at $0.2427 per kilowatt hour (kWh) regardless of when it is used. This rate is less than half what you would pay on the standard plan.​

It's a carrot and stick plan. They allow net metering in exchange for placing you on the plan. The plan gives you a lower per kWh price, but you pay for demand. This was the reason I installed powerwalls. In central Florida, it rains almost every day in the summer, and AC is a requirement for most people. In the winter, we produce almost 40% less energy. All it takes is one cloud at the wrong time to spike your bill $20 to $50. This plan nearly negates the benefits net metering, unless you add storage.

In the past I actively managed my powerwall to make sure I had enough energy stored to make it through the demand period while enjoying high demand devices like the HVAC, stove, and oven. One to two hours before the demand period began I would set the powerwall to self-powered mode with a low reserve. Then once the period passed, I would set the powerwall back to backup only with a reserve of 100%. I learned the hard way that the powerwall can take a long time to switch modes. Now I keep the powerwall in self-consumption mode and adjust the reserve.

I tried using Advanced Time Based Controls, but found that the behavior to be strange and unpredictable. The powerwalls would randomly send power to the grid when I would have preferred the batteries to be charged. Other times it would pull power from the grid even though the batteries were full. All of this happened during the defined peak periods. Then when in off peak, the powerwalls would continue to discharge until empty. After many calls to Tesla and their explanations not making sense, along with the documentation not matching up with actual behavior I gave up on Advanced Time Based controls.

I have gone back to changing the self-consumption reserve before and after the demand period. I'm wondering how others on demand plans manage their powerwalls, if at all. As a side note, we originally has a western facing solar array. I came to realize that the rain patterns in this area meant that we received less sunlight because it rained in the afternoon. We installed and additional eastern facing array and are now seeing how that affects our supply, consumption, and management of the powerwall.

We had a powerwall installed three weeks ago and I had all sorts of software issues for the first couple of weeks. One thing I didn't realize was that it takes an hour or two for a setting entered into the app to make it to the powerwall. I didn't figure this out until I used a computer to directly connect to the powerwall gateway via wifi and look at the settings using the web-based powerwall UI. I also had a situation where things would seem to work after Tesla would visit and tinker with the powerwall. But then I would play with the app and the powerwall would start to misbehave and show obviously incorrect data in the app (solar energy was correct, but home consumption was shown as exactly matching solar and grid energy was shown as zero which didn't match the electric meter or common sense). I could also get things to behave again by hitting the reset button on the gateway and then re-entering the wifi info and other stuff through the web-based powerwall UI. I was able to get self-consumption mode to work as long as I didn't try to change the reserve setting in the app. I also turned off storm watch just in case that was messing things up. Eventually I gave the Advanced time-based control a try (with the cost saving option) and much to my surprise it worked perfectly! I haven't changed the settings in the app since then and the powerwall now does exactly what I want it to given our time-of-use plan.

In the morning before peak prices start, all the solar is being used to charge the battery and the power to run the house is coming from the grid. When the battery fills up then the excess solar is sold to the grid. Once we hit 2 pm when peak prices start, the powerwall switches to using the battery to run the house and sells ALL the solar to the grid. Once the sun goes down, the house is powered exclusively by the battery until 9 pm when the peak prices end. Then the house switches back to the grid.

In order to qualify for the federal tax credit and/or to make our local utility happy it seems like the powerwall is only able to charge from solar and can only use this stored energy to power the house rather than selling it back to the grid. Under these constraints, the powerwall seems to be behaving optimally.

I can't be completely sure if rebooting the gateway + leaving the setting on the app alone is what fixed the problem. I also sent lots of emails to Tesla technician who installed the system and he escalated things to his manager so it's possible Tesla did something remotely to fix things.

The TL;DR version is to try rebooting the powerwall gateway, connecting directly through wifi to the gateway, and leaving the settings on the app alone as tempting as it is to tinker with things.

 

[Late2theGame]

I was looking at Tesla's Powerwall Support page and notice this under Time-Based Control / Energy Forecast:

Behavior
Powerwall will charge from excess solar during off-peak and shoulder (if required).
Powerwall will discharge during all periods, minimizing exports during shoulder and off-peak.

Powerwall Time-Based Control | Tesla Support

I don't recall seeing this before.

Actually, I believe most text on the whole page has changed ?! If so, might warrant it's own thread?

 

[DanAman]

That's new to me but describes the unwanted behavior I see when in time based control.

I was looking at Tesla's Powerwall Support page and notice this under Time-Based Control / Energy Forecast:

Behavior
Powerwall will charge from excess solar during off-peak and shoulder (if required).
Powerwall will discharge during all periods, minimizing exports during shoulder and off-peak.

Powerwall Time-Based Control | Tesla Support

I don't recall seeing this before.

 

[DanAman]

Once we hit 2 pm when peak prices start, the powerwall switches to using the battery to run the house and sells ALL the solar to the grid.

That is not the behavior I want. It wish Tesla would keep things simple and implement scheduled self-consumption before jumping into estimating my needs tomorrow. My observations are that they are not taking local weather conditions into account (today and tomorrow) when deciding how much to discharge. My area is looking at 6 days of cloudy/rainy weather. I have seen the powerwall drain itself in similar situations. That is a bad move for someone on a demand plan.

In self-consumption mode, the batteries do not seriously begin to discharge until 4pm for cloudy days and 6:30pm for sunny days. With 6 days of clouds and rain, time based control could leave me without enough to traverse the demand period.