Backing up EVSE - Am I overthinking this?

[Kodemonkey]

I have a 9.5kW system and 3 powerwalls on order. My main reason for the PW is for backup during a PSPS event (Planned Safety Power Shutoff - for those not in wildfire areas). Here is my thought process. Given:

1. When a PSPS is announced Tesla issues a storm watch to allow the PW to charge off grid - hopefully to 100%
2. If I am consuming 3kW and my solar is producing 7kW, solar will be shutdown. Now 100% of my power is coming from the PW until a point that the PW is at a far enough state of discharge to start charging and make up for the 5kW over-production.

If I had the EV on backup power, I could have it start to charge at a rate to push it over the 5kW overproduction. That way solar gets turned back on, the EV gets a little bit of charge, and the PW can start charging back up to 100%. This could be done manually or through interrogating the APIs and automating the charge rated through the Model Y API. I probably wouldn't spend the time to write the app though, I would do it manually.

What say you? Overkill? I also thought about just plugging in an electric space heater inside or outside (depending on the climate at the time), to push it over the "edge" for the inverters to come back on. But if I am going to have a Tesla EVSE installed, why not put it in the backup panel and let it top off the car and varying the charge rate if I can?

Or should I just throw an extension cord over my wall to my neighbors so they can plug in their fridges? Which I will probably do anyway, but that still might not get the solar turned back on.

 

[miimura]

Using your car to absorb excess solar production while the grid is down is a good idea. Supplying power to your neighbors is a good idea. Running a space heater for the heck of it when you don't need the heat makes no sense. Just let the solar remain offline until you can actually use it.

 

[gpez]

Just my $0.02: I did not back up my 14-50 EV charging plug but rather will use a 120v/12a charger to consume excess solar if the need arises. What I did do was configure my inverters to ramp down production as the Powerwall SoC increases. This will keep the Powerwall from charging/discharging (the see-saw effect) during a sunny day with the grid down and if I want to charge my EV I still can at 1.4kw.

 

[BrettS]

I think you may be misunderstanding exactly how and why the inverters get shut down while you are off grid. It doesn’t (directly) have to do with how much solar power is being produced or how much power is being used, but rather with the powerwall state of charge.

If the powerwalls are full (about 98% or above) when the power is shut off, then your inverters will be shut off no matter what. It doesn’t matter if they are producing 7kW and you are consuming 3kW of if they are producing 1kW and you are consuming 3kW.

Once the powerwalls power the house for a bit and their SOC is lowered (around 97% or less) then the inverters will come back on. If you are producing more power than you are using, then the powerwall SOC will go up and again, at about 98% the inverters will shut off. Things will continue cycling like this until the power comes back on or the sun goes down.

At this point you could potentially use excess solar power to charge your car. If you can keep the powerwalls in the mid 90’s then the inverters will stay on and you can use all the solar power that you can produce.

However, even without doing that the system will use as much solar power as it can to power your home and keep the powerwalls at a high SOC. The inverters will cycle on and off because without the grid and without being able to charge the powerwalls there is nowhere for the excess power to go, so some power will be ‘lost’. If you can send some of that excess power to your car or your neighbors, great, but there is no need to plug in a space heater and waste power just to prevent your inverters from cycling on and off.

It’s not like the system will keep your inverters off and totally drain your powerwalls just because your solar system is producing more power than you are consuming.

 

[jboy210]

Sounds OK. As the PWs get full, if you want to use every kW you end up running around like an old time movie looking for a bucket to hold the kWh coming off the solar. And EV is a good place to store this excess energy. I have used my Teslas for this.

Another thing is just because Storm Watch charging is enabled you don't have to let Storm Watch charge your batteries from the grid. You can turn it off and on in the app. I have done this to prevent pulling down power during high-cost periods, or if I am sure we are going to make it through the night without recharging the Powerwalls from grid (i.e billed) power.

 

[Kodemonkey]

Just my $0.02: I did not back up my 14-50 EV charging plug but rather will use a 120v/12a charger to consume excess solar if the need arises. What I did do was configure my inverters to ramp down production as the Powerwall SoC increases. This will keep the Powerwall from charging/discharging (the see-saw effect) during a sunny day with the grid down and if I want to charge my EV I still can at 1.4kw.

Ah. Didn't think about the wall charger. That may be enough to do it. I am going to read through your post - but I was under the impression that it was "on/off" and there was no scaling being done. Thanks for that info!

 

[Kodemonkey]

I think you may be misunderstanding exactly how and why the inverters get shut down while you are off grid. It doesn’t (directly) have to do with how much solar power is being produced or how much power is being used, but rather with the powerwall state of charge.

If the powerwalls are full (about 98% or above) when the power is shut off, then your inverters will be shut off no matter what. It doesn’t matter if they are producing 7kW and you are consuming 3kW of if they are producing 1kW and you are consuming 3kW.

Once the powerwalls power the house for a bit and their SOC is lowered (around 97% or less) then the inverters will come back on. If you are producing more power than you are using, then the powerwall SOC will go up and again, at about 98% the inverters will shut off. Things will continue cycling like this until the power comes back on or the sun goes down.

At this point you could potentially use excess solar power to charge your car. If you can keep the powerwalls in the mid 90’s then the inverters will stay on and you can use all the solar power that you can produce.

However, even without doing that the system will use as much solar power as it can to power your home and keep the powerwalls at a high SOC. The inverters will cycle on and off because without the grid and without being able to charge the powerwalls there is nowhere for the excess power to go, so some power will be ‘lost’. If you can send some of that excess power to your car or your neighbors, great, but there is no need to plug in a space heater and waste power just to prevent your inverters from cycling on and off.

Yes, I did not understand the threshold was 97%. I did understand about the excess power having nowhere to go, that was the reasoning to have the EV on the backup panel. Something to keep the "see-saw" effect from happening. But if all I need to do is keep the SOC at 95% and solar will stay on, I am good with that. If they won't let me put the EVSE on the backup panel then gpez' idea to use the 12A wall charger might do the trick. Thanks!

 

[jjrandorin]

You wont be able to set the powerwalls at a "SOC" of 95%. Its not like the cars where you can set a max charge percentage. You should be able to put SOME charging circuit in your backup panel. If you cant get your HPWC in there, you could ask them to put a 14-50 or even a 14-30 or some other circuit in there that would work from a load calculation percentage.

Thing on that is, you need to talk to tesla BEFORE the install team comes, because unless they already have it in the plans to install that outlet (or move the HPWC to the backup panel), they wont do it because it wont match the permit they pulled.

Based on what you said, you likely want to put some charging outlet on the backup side, and then in a power outage you set the new setting in the app that lets the car drain the battery to "a percentage". Set that percentage high, like 90% or so, and then let the car charge while your solar is generating, knowing that the car wont drain the batteries past your set percentage.

@BrettS did some testing on that setting if I remember right.

Anyway, the TL ; DR version is, you cant set max powerwall charge percentage, you set the reserve, but cant set a "dont charge higher than" percentage. To accomplish what you want, put either your HPWC or an outlet in the backup loads panel, and set the setting in the tesla app which limits how much the car can drain the battery in an outage.

 

[gpez]

And just to be clear: setting your PV to scale based on how full the Powerwall is to prevent see-sawing doesn't really help efficiency wise. There isn't a much of a difference between 6kw of solar for 10 minutes then off for 50 minutes vs 1kw for the whole hour. It's a tiny optimization as the sun sets but I like it because it prevents the frequency from getting much higher than 60.5hz.

Back to the topic at hand, if you have a TWC Gen 2 (and since your name and post imply you have at least some coding know-how) there are projects out there that can help you automate the car charging rate and keep you at that 95% sweet spot.

 

[BrettS]

And just to be clear: setting your PV to scale based on how full the Powerwall is to prevent see-sawing doesn't really help efficiency wise. There isn't a much of a difference between 6kw of solar for 10 minutes then off for 50 minutes vs 1kw for the whole hour. It's a tiny optimization as the sun sets but I like it because it prevents the frequency from getting much higher than 60.5hz.

For what it’s worth, even letting the inverters cycle on and off won’t let the frequency get higher than 60.5Hz either. Modern inverters cut out at 60.5Hz, so once the frequency hits 60.5Hz the inverter shuts off and the frequency doesn’t raise any higher.

Back to the topic at hand, if you have a TWC Gen 2 (and since your name and post imply you have at least some coding know-how) there are projects out there that can help you automate the car charging rate and keep you at that 95% sweet spot.

Tesla now allows you to do this natively. It only works while you are off grid, but you can set a minimum SOC level for the powerwall, such as 90 or 95% and it will work with your tesla car to regulate the charge rate so that it only charges with excess solar power and keeps the powerwall at the level you have set.

 

[jboy210]

Tesla now allows you to do this natively. It only works while you are off grid, but you can set a minimum SOC level for the powerwall, such as 90 or 95% and it will work with your tesla car to regulate the charge rate so that it only charges with excess solar power and keeps the powerwall at the level you have set.

So there is no way to do this when the grid is up? I would like to do that so I can divert power into the car rather than sell it back to PG&E.

 

[BrettS]

So there is no way to do this when the grid is up? I would like to do that so I can divert power into the car rather than sell it back to PG&E.

Right now the native car/powerwall interaction is a grid down only thing. I know that a lot of people have requested that it be available at all times, but I don’t know if that’s on Tesla’s roadmap or not.

 

[AD3T]

+1 -- I'd like to be able to do this, too. I receive only 80% net-metering credit -- I'd rather top-up my Tesla, up to 75%, before beginning to export, simply for the sake of it being the higher efficiency usage/storage of that energy [only slightly, after accounting for losses, but even still is better].

I thought that I read that someone had setup some custom code via API to "manually" match their vehicles charge-speed to their current "over-production" to essentially achieve exactly this; maybe I imagined it, but could have sworn I saw a post either here or on Reddit that someone had managed to do so.

Though now I'm doubting myself, as I'm not sure that the API even has any endpoint to set vehicle charge-rate (it's not in the Tesla app, for instance); perhaps the latest version Tesla wall-chargers, which are wifi-connected IIRC, provide the ability to set charge rate via API? Not certain, but since I have the prior version of the wall-charger (non-wifi) I'm unsure if there's even a way for me to set vehicle charge-rate without me physically being in the car to adjust it via the touch-screen. Anyone happen to know?

 

[sorka]

With 1.50.1, my powerwalls when off grid output exactly 60 Hz and didn't start edging up until after passing 97%. By the middle of 98%, they'd hit 60.4 and the inverter would shut off.

With 1.50.2, the PWs will output 60.4 hz at 82% when charging with 6KW. Solar shuts off and then restarts 5 minutes later and runs for about 5 minutes and shuts off at a slightly higher SOC than before. As the SOC climbs, it takes less and less power to trigger the PWs into climbing to 60.4 Hz.

By the time they hit 89%, it only takes 3.4 KW to trigger them to rise to 60.4 Hz and 2.7 KW at 91%.

I discovered that as the frequency rises, if I put enough load on to reduce what was going to the PWs below these thresholds, I could keep the PWs from shifting the frequency up.

When turning on the grid briefly during peak solar production at high SOCs, the powerwalls will gladly accept the full solar output even at 90% or more. If the above behavior was some sort of protection from being charged too quickly, you'd think that on grid, the power the TEG doesn't want to send to the PWs would just go to the grid but the TEG gladly sends all solar production to the PWs.

What's also not great is that the PWs now always output 59.6 to 59.7 Hz unless they are being triggered up in the scenarios described above resulting in my Insteon network becoming unreliable at ANY SOC while off grid. None of this was a problem with 1.50.1.

The third peak of solar production just after midday is fatter because the grid was re-attached for a little while. As soon as I turned the grid back off, the PWs shot up to 60.4 Hz within a few minutes shutting the inverter off.

vs a similar day on 1.50.1

 

[Kodemonkey]

All great information, thank you. I've been off the forum lately (picked up my LR Model Y yesterday and have been interrogating the API). I am waiting until after the solar is installed and have PTO before I have the wall connector installed. Thankfully, they didn't listen to me and they are moving a 50A 2 pole breaker to the backup panel that was for a spa but has since been removed. Why they would think it would be important to me to have a spa on backup power is beyond me, but I digress. So Tesla Energy accommodated for the 50A spa in the load calculation and they even overshot my square footage by 50% and it still came under. I'll move it up to a 60A when I have the wall connector installed after I have PTO.

 

[gpez]

With 1.50.1, my powerwalls when off grid output exactly 60 Hz and didn't start edging up until after passing 97%. By the middle of 98%, they'd hit 60.4 and the inverter would shut off.

With 1.50.2, the PWs will output 60.4 hz at 82% when charging with 6KW. Solar shuts off and then restarts 5 minutes later and runs for about 5 minutes and shuts off at a slightly higher SOC than before. As the SOC climbs, it takes less and less power to trigger the PWs into climbing to 60.4 Hz.

By the time they hit 89%, it only takes 3.4 KW to trigger them to rise to 60.4 Hz and 2.7 KW at 91%.

I discovered that as the frequency rises, if I put enough load on to reduce what was going to the PWs below these thresholds, I could keep the PWs from shifting the frequency up.

When turning on the grid briefly during peak solar production at high SOCs, the powerwalls will gladly accept the full solar output even at 90% or more. If the above behavior was some sort of protection from being charged too quickly, you'd think that on grid, the power the TEG doesn't want to send to the PWs would just go to the grid but the TEG gladly sends all solar production to the PWs.

What's also not great is that the PWs now always output 59.6 to 59.7 Hz unless they are being triggered up in the scenarios described above resulting in my Insteon network becoming unreliable at ANY SOC while off grid. None of this was a problem with 1.50.1.

The third peak of solar production just after midday is fatter because the grid was re-attached for a little while. As soon as I turned the grid back off, the PWs shot up to 60.4 Hz within a few minutes shutting the inverter off.

vs a similar day on 1.50.1

So I don't know about the behavior differences you're seeing with the firmware version changes but when I did my testing last year on v1.37.1 I noticed that when off grid my local frequency changes based on both the Powerwall SoC but also the house load.

Roughly:

High SoC, low load = 62hz
Low SoC, low load = 60hz
High SoC, high load = 61hz
Low SoC, high load = 60hz

From my experience the higher weight factor is the SoC and load plays a smaller role. Perhaps 1.50.2 exaggerates this behavior?