Unwanted grid draw during solar charging. Is this normal?

[CaptShazbot]

My Settings
Brand new solar and Powerwall install, self-powered mode, 30% reserve, grid charging off

Symptoms
In the morning, when solar production starts and the battery is above reserve, I see some odd behaviour in the screenshots attached.

The documented behaviour for my settings is for the solar to prioritise the house, and charge the battery with excess solar. What I'm seeing is bursts of 0.1 kW drawn from the grid to supplement power to the house, even though the solar is producing enough on its own. It sort of alternates between drawing from the grid and not drawing from the grid. The graphs for this time period look a bit funny, almost like a gap where I should see more home usage.

This is the only time I see errant grid usage: that window of time during charging and solar production. When solar production has stopped and the battery is discharged to the home, everything continues to work as it should.

I've reported this to Tesla and it has been escalated, but I was wondering if anyone else sees something similar? I did find the thread below, but this seems to be a fairly edge case with an odd system design. My system is doesn't share those issues as far as I know.
Solar pulling from grid vs Powerwall [in small amounts]

I realise this total grid use is fairly small, but I would like to get to the bottom of whether there's a misconfiguration in my system.

 

[ptdusn]

Yes, I had the same issue when Sunrun replaced my Neurio meter. Turned out they had the meter and the gateway set as "site" when the neurio should not have been set as that. have Tesla check those settings.

 

[CaptShazbot]

Thanks! I don’t have a Neurio meter, but can you clarify if the Gateway should or shouldn’t be set as site? Is this something Tesla Support can check remotely?

 

[CaptShazbot]

Update
I’ve discovered this issue seems to manifest only in self-powered mode. When toggling to time-based control, the grid draw stops. I can make it start and stop reliably just by toggling the modes back and forth.

Note that the issue happens during peak pricing, so both time-based control and self-powered mode should behave the same (not draw from the grid).

My thinking is that this may point toward a firmware bug or misconfiguration. I’ve requested Tesla push a firmware update to the Gateway, and I’ll re-test the following morning. Otherwise, it is still in the queue for Tier 2 Support to investigate.

 

[ptdusn]

Thanks! I don’t have a Neurio meter, but can you clarify if the Gateway should or shouldn’t be set as site? Is this something Tesla Support can check remotely?

Gateway set as site.

 

[CrazyRabbit]

I get that too. I’ve just chocked it up to the heater running in powerwall.

 

[ptdusn]

I get that too. I’ve just chocked it up to the heater running in powerwall.

heater in powerwall runs off of internal battery supply.

 

[CrazyRabbit]

Don’t know if heater is ac or dc. Anyone know the draw of the heater.
I would think the heater is ac, needs to heat before it can charge.

 

[ptdusn]

Don’t know if heater is ac or dc. Anyone know the draw of the heater.
I would think the heater is ac, needs to heat before it can charge.

Can be DC.

Preconditioning​

When temperatures are low, all batteries have a reduced ability to charge. To help Powerwall counteract this, Powerwall uses Preconditioning. When temperatures are below freezing, Preconditioning turns on and heats your Powerwall to improve operation and charging performance. To heat itself, Powerwall draws a small amount of energy, which then allows high-power charging.
During a cold night, your Powerwall automatically preheats before sunrise so that maximum solar energy can be captured during the day. Preconditioning, in combination with Tesla’s unique liquid thermal management system, allows your Powerwall to operate at lower temperatures than any other home battery.

Additional considerations:​

• You cannot customize this mode because Powerwall can best detect its internal temperature and knows when to enable Preconditioning.
• During Preconditioning, the Power Flow screen may show energy flowing to your Powerwall from solar or the grid. This is normal behavior, and the energy is only being used to heat, not to charge.
• During winter months, a small amount of your Powerwall capacity is reserved to improve performance in cold weather. This may change your visible total capacity in the Tesla app. Rest assured, your total capacity has not changed — this small reserve is just set aside to improve performance.

 

[Evil-The-Cat]

Few things. I pulls some power every now and then as its syncing with the grid, there is also a little CT error that means at low loads it's not totally perfect.

It also doesn't react instantaneously to load changes, watch what happens when your microwave is running or another large load and then stops, it pulls from the grid for a second at the start before the PW catches up, and it exports for a second at the end when the load stops but the PW is still discharging. It is tiny tiny amounts so shouldn't have any impact on your bill. I think it cost me a total of 6p of peak power on last months bill.

 

[CaptShazbot]

Thanks for the replies.

CT error and reaction time make more sense to me than preconditioning, for my situation, because Powerwall is indoors in mild temperature.

One other thing I’ve noticed is that when not charging, the two Powerwall operating modes use a different amount of constant grid draw. Self-powered pulls 15-20W, while time-based control only pulls 4-6W. This is during peak pricing when both modes are effectively doing the same thing and fully powering the house. This difference is pretty consistent in my testing.

 

[CaptShazbot]

Update

Tesla has closed my support ticket, saying that the grid draw is just transient loads. I disagree.

I’m seeing a very clear difference in behaviour between self-powered mode and time-based control mode when Powerwall is charging:

Self-powered — Powerwall always takes 0.1kW more than it should to charge from solar, and therefore the house draws a constant 0.1kW from the grid to make up the shortfall. This is not documented behaviour.

Time-based control — Powerwall only takes excess solar to charge itself and therefore there is no grid draw. Correct behaviour.

So effectively, during peak pricing when Powerwall is not full, TBC does a proper job of being self-powered, and self-powered mode doesn’t.

I want my home to be self-powered and to minimise grid usage. This leaves me in the situation of having to manually change to TBC every morning, then change it back to self-powered every night before off-peak pricing starts

It’s pretty irritating and I’m not sure how to get it fixed. It seems like a firmware issue, but no one else seems to be experiencing it.

 

[CaptShazbot]

My issue has been fixed. I thought I'd write a final update for posterity, in case anyone ever runs into this. For once it's not a CT issue.

It turns out my Powerwall was incorrectly configured with the setting PV Export Limit: 0 kW. If you log into the Gateway web interface as a customer, the setting appears like this on the Summary page:

summary.png

Note that this setting doesn't prevent the PV system from exporting — my system was exporting to the grid just fine — it just tells the Powerwall that the PV is not allowed to export.

What that does is cause an undocumented but intentional behaviour by the Gateway. As explained to me, it "fakes" a constant 0.1 kW draw from the grid to trick the solar inverter into not curtailing solar production, by making it think there is always unfulfilled demand onsite. That is, if a PV system truly cannot export, it may mistakenly curtail its production when the Powerwall is discharging to the house, because it wouldn't be able to understand there is demand there — it sees the battery and house as a combined load. I hope that makes sense. However this grid behaviour works, it not only affected reporting in the Tesla app, but it was also registered by the utility meter and billed to me.

With the erroneous export limit removed, the unwanted grid draw was immediately fixed. For the first time, I've been able to achieve 100% self-powered.

 

[CrazyRabbit]

Can you post your picture here, Dropbox don’t work for me.

 

[BGbreeder]

You don't need a Dropbox account to see it, but here is @CaptShazbot's image

 

[CaptShazbot]

You don't need a Dropbox account to see it, but here is @CaptShazbot's image

Thanks for posting the image inline. I should've done that in the first place.

 

[BGbreeder]

Thanks for posting the image inline. I should've done that in the first place.

No worries! You are welcome. (And a belated welcome to TMC!)
All the best,
BG