Can Powerwall charge from solar and export to the grid?

[MikeW]

I'm on solar with the EV-A rate plan. I got a letter from PG&E that this rate plan is ending and I will be moved to EV-2A. Unfortunately the time of use periods for EV-2A are much less favorable for net metering:
Peak: 4:00 pm to 9:00 pm
Part Peak: 3:00 pm to 4:00 pm and 9:00 pm to 12:00 am
Off Peak: All other hours (i.e. 12:00 am - 3:00 pm)

I'd like to charge the Powerwall from sunrise until 4:00 pm and then fully discharge from 4:00 pm to 9:00 pm, regardless of my power consumption at my house. Is that supported? I can't tell from the support pages if "Cost Saving" mode does that.

 

[power.saver]

Not quite. You can fully charge anytime the sun is out, but you cannot discharge to the grid, ever. What you can do is discharge to offset your house load, and send all solar to the grid. That is what I do using Time Based Control - Cost Savings.

 

[RoBoRaT]

Like what power.saver stated. PW will not discharge to the grid.

My set-up:
Time Based Control - Balanced.
PWs 50% reserved for outage.

Off Peak: 10 PM - 8 AM $
Semi Off Peak: 8 AM - 2 PM $$
Peak: 2 PM - 10 PM $$$$

10 PM:
Grid feeds Home/EV from 10 PM - 8 AM
Charge EV at 1 AM.

8 AM:
PW starts to feed Home.
Solar takes over, recharges PW to 100%, then sends excess to Grid.

2 PM:
Solar sends all to grid, while PW feeds Home until 10 PM.

10 PM:
Grid feeds Home/EV. Rinse, Repeat.

So far - PW is working as described by Time Based Control.

 

[wwhitney]

There's a couple issues with what you (the OP) propose.

First is the interconnection agreement you have with the utility. Even if you have solar, sometimes (often?) the interconnection agreement governing the Powerwalls is a "non-export" agreement. That means your Powerwalls aren't supposed to export to the grid, even though your PV system is allowed to export.

If you don't have this constraint, then I believe that means the utility is fine with using your battery for the purpose of solar generation time shifting. The problem then is that Powerwall's software doesn't support this operation mode. I don't see any technical or ITC reason Tesla shouldn't implement this, but perhaps there are political reasons precluding it.

So if you want to do this today, the only option would be to trick the Powerwall into doing what you want. The idea I have in mind is to spoof the signal to the CTs on the Neurio in the Gateway to make it look like your consumption is higher than it actually is. This spoofing would be controlled to only occur during Peak hours.

My basic idea is to add another CT in parallel to the Neurio's CTs and to run a dummy load through it. The Neurio's CTs are labeled 264A/88mA (3000:1 turn ratio) and to my knowledge are a current output (no built-in burden resistor). So as I understand it if a current output CT with a much lower turn ratio like 10:1 is hooked up in parallel to it, the current outputs will add, and the Neurio will read the current through the new CT as a factor of 300 higher than it actually is. That is, you could use a 3W dummy load and the Neurio would see it as an additional 900W load.

That's as far as my thinking has evolved, I'm concerned I've reached the point of not knowing what I don't know. There may be problems I'm unaware of with parallel CTs with different turns ratios. CTs have a "VA burden" rating that I don't understand. One person claims the Neurio uses an internal 1 ohm burden resistor, for what it's worth.

So if anyone with more of an electrical engineering background wants to help me figure this out, please PM me.

Thanks,
Wayne

 

[MikeW]

Even if you have solar, sometimes (often?) the interconnection agreement governing the Powerwalls is a "non-export" agreement. That means your Powerwalls aren't supposed to export to the grid, even though your PV system is allowed to export.

If you don't have this constraint, then I believe that means the utility is fine with using your battery for the purpose of solar generation time shifting. The problem then is that Powerwall's software doesn't support this operation mode. I don't see any technical or ITC reason Tesla shouldn't implement this, but perhaps there are political reasons precluding it.

This is definitely allowed in California -- a CPUC decision early this year says NEM customers can either do "No Grid Charging" or "No Storage Export".

CPUC Approves Energy Storage Net Metering

Your idea is intriguing but ideally Tesla would add support for storage export. What is the best way to make a feature request?

 

[kevinkeenan]

I’m on National Grid in MA. I just signed up for a service this summer where during high peak times the power company draws power from our (and other homes) batteries. We are paid for these events and they are like the opposite of Storm Mode. We have no control over these events other than to allow or not allow. If Storm Mode activates, however, the grid can reverse and charge the battery back up.
I know it’s not exactly what you are looking for, but at least proves that sending power the other way is possible.

 

[SongjinDK]

Thank you all for this information. I just ordered 2 powerwalls for this purpose, looks like I should cancel the order.

One idea is to buy a Nissan Leaf or even Tesla battery and build a DYI PW to discharge to the grid. Is this feasible?

 

[miimura]

Thank you all for this information. I just ordered 2 powerwalls for this purpose, looks like I should cancel the order.

One idea is to buy a Nissan Leaf or even Tesla battery and build a DYI PW to discharge to the grid. Is this feasible?

Are you trying to install a system with all the proper permits and permissions in place from the city and utility?
If so, you need to look up which inverters are approved by the utility for interconnection. You should also inquire with the about what the export rules are. In my case, with Powerwalls permitted as 'Parired Storage' with PG&E, the billing system explicitly disallows NEM credits in excess of your calculated solar production. See the thread below.

PG&E Letter - Paired Storage Billing Update

 

[MikeW]

So if anyone with more of an electrical engineering background wants to help me figure this out, please PM me.

I'm a software engineer but I do have an EECS degree.

Low cost CTs often output AC voltage proportional to the current they are sensing. If the Neurio CTs are like that, putting more CTs in parallel wouldn't work. Instead you could wire additional CTs in series, or you could use an amplifier to increase the voltage.

 

[markb1]

This is definitely allowed in California -- a CPUC decision early this year says NEM customers can either do "No Grid Charging" or "No Storage Export".

CPUC Approves Energy Storage Net Metering

Your idea is intriguing but ideally Tesla would add support for storage export. What is the best way to make a feature request?

I don't really get what that article is saying, because NEM already allowed exporting stored solar power. You just can't export more then your solar panels generate.

It's all spelled out here in the PG&E NEM tariff (section 11): https://www.pge.com/tariffs/tm2/pdf/ELEC_SCHEDS_NEM.pdf

And it's essentially the same for SDG&E and SCE.

 

[wwhitney]

Low cost CTs often output AC voltage proportional to the current they are sensing.

Yes, some CTs have a built in burden resistor. However, as I mentioned, the ones Neurio use do not.

I'm confident that two of the Neurio CTs could be put in parallel without causing any problems; in fact I believe some of the installation threads mention that this was done. My concern is about paralleling a Neurio CT with a different manufacturer's CT, one with a much lower turns ratio, and whether this could entail complications I'm unaware of.

Cheers, Wayne

 

[MikeW]

I don't really get what that article is saying, because NEM already allowed exporting stored solar power. You just can't export more then your solar panels generate.

The summary mentions "large facilities", which means over 10 kW (3 or more Powerwalls). Maybe it was allowed before for small residential. It also removes the requirement for a net generation output meter (NGOM) which lowers the installation cost.

Summary
This decision partially grants a petition for modification of Decision (D.) 14-05-033, to allow large net energy metering-eligible facilities paired with energy storage using alternating or direct current configurations to take service under a Net Energy Metering Tariff. In D.14-05-033 the Commission declined to consider a possible pathway for direct current configurations, acknowledging such configurations may not be able to accommodate the metering equipment required by D.14-05-033. This decision approves non-metering, power control- based options for ensuring net energy metering credit accrues only the net energy metering-eligible generation, as long as the control configuration is certified to a national standard or a utility-approved interim testing procedure. Power control-based options include the use of equipment, whether firmware- based or software-based, to prevent the storage device from charging from the grid or to prevent the storage device from exporting to the grid.

 

[MikeW]

So if you want to do this today, the only option would be to trick the Powerwall into doing what you want. The idea I have in mind is to spoof the signal to the CTs on the Neurio in the Gateway to make it look like your consumption is higher than it actually is. This spoofing would be controlled to only occur during Peak hours.

I have Powerwalls on order, so I can't try this yet, but this web site says

There are other fun CT Clamp Flip Combinations. Another combination forces the PW to dump it's entire charge back into the grid

via the api - which is exactly what we want.

hackerschoice/thc-tesla-powerwall2-hack