Told my Powerwall won't charge off Solar

[GasDoc]

My free Powerwall, from the referral program, is being installed this week; after five years of waiting!

We are installing at a home that has an existing, owned, 12kW solar array. The array is at least ten years old so I imagine it's degraded some and not really 12kW anymore.

Tesla called me and informed me that because of the size of my array, my Powerwall will not be able to charge off solar. It will be installed only to charge off the electrical grid.

The Tesla energy technical guy told me that to charge off solar, I would need another Powerwall and a new design, permits, etc.

I told them to proceed with the installation and I might add that later, knowing that it would cost me an additional installation fee. I want the "bird in the hand" after waiting this many years.

I'm not super familiar with the design of solar energy systems. Can someone explain why the solar can't just charge my PW to full and then send the additional energy to PGE net metering? I'm ok if some efficiency is wasted and not all the energy is captured while charging the PW because the inverter can't take advantage of the entire 12kW array.

 

[jjrandorin]

Can someone explain why the solar can't just charge my PW to full and then send the additional energy to PGE net metering?

The high level short answer is because you likely have too much solar energy generated at once for one powerwall to handle.

FWIW (which isnt much), If I were you there is no way I would accept an install of 1 powerwall "only connected to the electrical grid" when I had solar. You probably think. "I will just get another one later" and the cost of that will probably be waaaaayyyyy (way) more than you think its going to be.

You should take the free powerwall, buy a second, and have them installed so that it works properly. You should get some install credit for the free / referral one, so it will never (ever) be as cheap for you to install another powerwall along with the one you are getting as it will be right now.

 

[GasDoc]

The high level short answer is because you likely have too much solar energy generated at once for one powerwall to handle.

Can't the extra energy just be wasted somehow?

 

[jjrandorin]

Can't the extra energy just be wasted somehow?

No, because if you are in a power outage it has to have somewhere to go. Its possible that your entire solar install could be re configured to make only a part of it active during a power outage, but that would likely require a bunch of re work of your existing PV system, if its even possible.

So, no, not really.

 

[patrick40363]

Buy a second Powerwall and have it installed at the same time. Your existing solar will work fine with the 2 powerwalls,

 

[GasDoc]

Buy a second Powerwall and have it installed at the same time. Your existing solar will work fine with the 2 powerwalls,

This would require a redesign and new permits, likely months of delays. I waited years for Tesla to come through with this free Powerwall from the referral program. I'm going to go with installing now and paying the penalty down the road of an additional installation fee when I purchase a second Powerwall.

 

[jjrandorin]

This would require a redesign and new permits, likely months of delays. I waited years for Tesla to come through with this free Powerwall from the referral program. I'm going to go with installing now and paying the penalty down the road of an additional installation fee when I purchase a second Powerwall.

Just as long as you know that adding that second powerwall "later" is likely to cost you somewhere around 12-14k ($12,000 to $14,000) just the same as if you didnt have one at all, where if you do it right now it will probably be the cost of just the powerwall itself. So, probably double the cost to do it "later".

 

[GasDoc]

Just as long as you know that adding that second powerwall "later" is likely to cost you somewhere around 12-14k ($12,000 to $14,000) just the same as if you didnt have one at all, where if you do it right now it will probably be the cost of just the powerwall itself. So, probably double the cost to do it "later".

Yeah. The guy said it would be an additional $8500 to add the second Powerwall, redesign the system, wait for permits, and install two walls at once. To add it later would be the same $8500 for hardware and about $3k for installation costs. So I pay a three thousand dollar penalty to install the second Powerwall later.

But given my experience of having to wait this many years for this first (free) Powerwall, I just want to get it done and deal with the second one later. The purpose of this thread was to try to get an understanding of why I can't charge off solar with just one unit.

 

[wwu123]

I don't have a Powerwall, so not intimately familiar with the various config modes with or without solar, but isn't there a TOU configuration that can charge during off-peak from the grid, which is essentially the same cost as charging from solar off-peak anyways, aside from possible non-bypassable charges? I'm sure if you can configure a TOU off-peak period to be charging in the late morning, it is effectively charging from solar before the excess solar goes to the grid anyways, since electrons are electrons?

 

[GasDoc]

I don't have a Powerwall, so not intimately familiar with the various config modes with or without solar, but isn't there a TOU configuration that can charge during off-peak from the grid, which is essentially the same cost as charging from solar off-peak anyways, aside from possible non-bypassable charges? I'm sure if you can configure a TOU off-peak period to be charging in the late morning, it is effectively charging from solar before the excess solar goes to the grid anyways, since electrons are electrons?

The major downside of my configuration is that if the grid power is down, my reserve is limited to the capacity of one powerwall. Solar won’t charge it. On the other hand, if I could charge off solar and had sufficient battery and sunshine, I could weather through an indefinite power outage; essentially, being off the grid.

 

[cali8484]

Can someone explain why the solar can't just charge my PW to full and then send the additional energy to PGE net metering?

There should be no issue with only 1 PW while connected to the grid. Any excess solar would simply flow to the grid. Off-grid is a different story. You should ask for more clarification. Alternatively, you can use OpenEVSE or Emporia to get charge on solar functionality while connected to the grid.

 

[lodar]

Can't the extra energy just be wasted somehow?

I am surprised as well, because the Powerwalls can as the solar to “curtail” their power by modulating the power frequency.

Maybe your system doesn’t support that? Or isn’t configured to support frequency-watt curtailing? It can be a PITA to get the solar people to configure the right profile on the inverters.

During a scheduled outage in the summer, I cranked the AC to keep the Powerwalls from filling up. But as they did, the gradually increased the power frequency according to my UPS, and the solar generated significantly less power. I started cooking on my electric stove, the Powerwalls’ frequency went back to 60 Hz (or even 59.9 Hz), and solar was at full power again.

 

[jjrandorin]

I am surprised as well, because the Powerwalls can as the solar to “curtail” their power by modulating the power frequency.

There is very little (almost no) chance that a 10 year old solar inverter is going to support anything other than "on / off" as far as solar generation goes. A system that age wont curtail.

 

[GasDoc]

There is very little (almost no) chance that a 10 year old solar inverter is going to support anything other than "on / off" as far as solar generation goes. A system that age wont curtail.

I think my Powerwall is supposed to come with an inverter. But I don't understand these things much. Maybe I just need an upgraded inverter for my solar array to bridge the technology gap?

 

[miimura]

It would help if you told us what solar inverter(s) are used in the old solar system. The best solution for your situation is to put part of the solar system on the backup side and part on the grid side of the Gateway. That way you avoid the problem of overloading the Powerwall with too much generation when the grid is down.

 

[arnolddeleon]

You mentioned your array size but not the size/quantity and model of your existing solar inverters.

12kW of solar panels 10 years ago has a decent chance of being on multiple inverters. You might be able to split your PV inverters if you have more than one.

The theoretical problem is the PV system will produce more solar (net of any loads that you have running) than the Powerwall can absorb when partially empty, about 5 kW while the grid is down. But in practice, your PV system will not be net over 5 kW a lot of the time. When it is "over producing" the Powerwall can signal the PV inverters to shutoff. I think Tesla is just trying to avoid the support calls from homeowners wondering why their Powerwall is not charging when the grid is down. But they already have to deal with situation of shutting the solar down even when they are sized "small enough" because the batteries can be full.

If you have multiple inverters and they were all behind the Powerwall you could theoretically manually shutdown part of your system.

About five years ago, I was able to convince Tesla to install only two Powerwalls when their calculations at the time said I needed three because of the size of my PV system.

 

[miimura]

The rule of thumb is that you should have no more than 7.6kW AC of solar inverters per Powerwall 2 on the backup side of the Gateway.

 

[zƬesla]

The rule of thumb is that you should have no more than 7.6kW AC of solar inverters per Powerwall 2 on the backup side of the Gateway.

Our initial install by a Tesla certified installer was for 14.7kW total (2 inverters) and 2 Powerwall2s as they also believed 7.6kW per PW. Long story short, they installed a 3rd PW2 at their cost. 5kW per PW2 seems appropriate.

 

[BGbreeder]

Our initial install by a Tesla certified installer was for 14.7kW total (2 inverters) and 2 Powerwall2s as they also believed 7.6kW per PW. Long story short, they installed a 3rd PW2 at their cost. 5kW per PW2 seems appropriate.

The upper charging limit for a Powerwall is 5kW, and preferred charging rates seem closer to 3kW.

I think the 7.6kW idea came (comes?) from factoring in less than ideal roof slopes and orientations that would cause many 7.6kW installations never, or almost never, to exceed 5kW of output power. @zƬesla I suspect that your adjustable mounts enable you to routinely achieve higher actual power outputs, and led to your Powerwall charging issues and the extra Powerwall.

@GasDoc I hope that this thread helps you. I can understand your desire to finally get your hands on your free Powerwall, but personally, I would go for the second now, as I suspect that it will cost more than $3k extra in the future, as backward compatibility in the whole solar / storage area seems to be mixed.

All the best,

BG

 

[zƬesla]

I think the 7.6kW idea came (comes?) from factoring in less than ideal roof slopes and orientations that would cause many 7.6kW installations never, or almost never, to exceed 5kW of output power. @zƬesla I suspect that your adjustable mounts enable you to routinely achieve higher actual power outputs, and led to your Powerwall charging issues and the extra Powerwall.

@BGbreeder - The installer admitted to misunderstanding Tesla guidance and that they should have used 5kW per PW2, that's why they comp'd one. The panels being pole-mounted is irrelevant. Any inverter(s) capable of producing > 5kW per PW2 is likely to cause the GW to shut down the system during peak production which is what we saw happening. Cloud edge effect may be enough to cause it to happen even with less than ideal slopes. At least that's my understanding after having spent way more time on the issue than I wanted