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PowerWall w/ Solar, Charge From Grid Experiment

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May 12, 2020
Oakland, CA
I have become curious about how charging a PowerWall from the grid works. PW does this when Storm Watch is active, and Tesla reportedly does grid charging in non-solar instllations, and reportedly both grid and solar charging in some countries. I wonder wow Tesla controls this, and how does it function when it is allowed?

The issues around grid charging, such as regulations, tax issues , enconomics, and so on, are a topic for some other thread. Here, lets please stick to the technical issues.

So I did an experiment. While nothing bad happened, I do not recommend doing this. Accesing the installer interface is explained elsewhere, so I won’t go into that.

Here is what I did: Using the installer wizard I set the PW to No Solar, and set the two solar CT’s to “Site”, the same as the setting for the standard CT’s on the grid connection, so that PW would correctly calculate the house load. More on that below.

When I put the PW into Backup Only mode, it did charge from the grid, just like it does when Storm Watch is active. However, for several reasons, this clearly is not how Tesla configures PW for grid and solar charging.

The way the PowerFlow numbers changed is sensible, but not what is disired. The House and PW flows displayed accurately, and it showed no sloar production, as expected with no CTs set to report Solar. The Grid number however is a bit stranger - it now showed the sum of the actual solar and the net grid flow, as one would expect. But when the PW was not charging and the solar was putting out more than the house was using, power was naturally going out to the grid. With both the solar and grid CT’s set to detect Site power, both are added to make the displayed Grid flow. So the negative grid flow was offset by the solar flow resulting in a net flow equal to the house flow. The House number is actually not measured directly, but rather calculated as the sum of grid, solar and PW flows. So it displayed as one would expect given the settings, but there was no display of the actual solar production.

So, this is clearly not how Tesla configures it. The App should display and track the solar production.

Another issue is that the PW did not charge when I put it into Cost Savings mode. Normaly it would have been charging from solar, but without knowing there was any solar PW did not chare at all. Maybe it needs to relearn the new flow patterns, I don’t know.

Most importantly however, I did not test a grid outage. This might be problematic due to grid failure detection issues, grid flow being conflated with solar, shutting down solar when fully charged, and whatever I haven’t thought of. So I didn’t try that. I expect Tesla designed in all sorts of protections for when installer’s make configuration errors, but that was not what I wanted to explore.

If somone needed to use this tehcnique in a pinch to charge off the grid, I would suggest turning off the solar first. Perhaps in that case, setting the solar CT’s to “None” would behave better, but I did not try that either.

Apparently Tesla has the ability to remotely adjust parameters in PW that we can not see or adjust in the installer wizard. For example, I had them adjust the frequency shift which interfered with my router’s unitteruptable power supply, so that is one such hidden parameter. Grid charging must involve other such hidden settings.

If anyone has experience or knowledge about how PowerWall’s do function when allowed to charge from the grid, or how to get PWs to do so, I’d like to hear about it.

Again, let’s please discuss the why’s and why-not’s in a different thread.

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