11.6kw Solar Roof, but only one Inverter?

[Vines]

Nothing is out of the ordinary here, except things are working as designed and the answers were written in your post:

It seems like I might be the first person to offer some evidence that the whole Overdrive idea might be real!

Background: I live in Maine and I just had my PV and Powerwall system installed by a third party. I have 10.8 kW of Solar panels, One Powerwall+, and one Powerwall. They are connected to my system with a Backup Gateway 2. My Powerwall+ has a single 7.6 kW inverter.

Last Tuesday, I was looking at my Tesla app and the system was currently generating 11 kW of energy. Here were the specific conditions:

- I was charging my Model Y so the house was using 16.7 kW
- It was a perfectly clear, cold day and my yard is covered in snow. The snow must have bounced additional light into the panels to exceed their 10.8 kW rating
- I currently have permission to export OFF

If you look a the inverter datasheet, it does specify that it can output 32A (7,6 kW) on grid, or 40A off grid. With your permission to export set to off, and your home loads utilizing more than 10 kW, I suspect the full 40A (9.6 kW) can be generated and used by your loads. The above numbers are at 240 VAC, but more power can be produced if the grid voltages are a bit higher. I do not know if this accounts for your 10.8 kW however, and I suspect your PV CT's might be a bit optimistic. I personally haven't seen a system perform to the nameplate rating of the DC STC system size, except for very brief periods.

You can verify most of this information from the screenshots attached below. I didn't think it was possible to exceed the inverters 7.6 kW rating, and my understanding is that the inverter and the Powerwall do not have a direct DC connection, only AC. Maybe that's incorrect, and the inverter is sending the excess DC voltage to the Powerwall to be utilized? Also, my system hasn't clipped once in my three weeks of having this system.

I'd love to hear everyone's theories as to how this is all possible. However it's occurring, I'm so glad I have the Powerwall+ system versus using a different inverter that can't go past it's inverter rating. The sun is shining again today and my system is generating 8 kW as we speak.

I will be interested to hear your reports once your system gets PTO, and what solar production you will see while on grid

 

[Josh Gerritsen]

Nothing is out of the ordinary here, except things are working as designed and the answers were written in your post:

If you look a the inverter datasheet, it does specify that it can output 32A (7,6 kW) on grid, or 40A off grid. With your permission to export set to off, and your home loads utilizing more than 10 kW, I suspect the full 40A (9.6 kW) can be generated and used by your loads. The above numbers are at 240 VAC, but more power can be produced if the grid voltages are a bit higher. I do not know if this accounts for your 10.8 kW however, and I suspect your PV CT's might be a bit optimistic. I personally haven't seen a system perform to the nameplate rating of the DC STC system size, except for very brief periods.


I will be interested to hear your reports once your system gets PTO, and what solar production you will see while on grid

Thank you everyone for chiming in! Lots to respond to, so I'll just do it all in this reply.

taphil: I don't have PTO yet so I can't run that test. In terms of mentioning overdrive, I'm referring to ericl at the very beginning of this thread saying that his Tesla Advisor said it was possible.

jgleigh: Damn, you're right! I just confirmed from page 55 of the Powerwall+ installation manual that they shouldn't have added that CT on the 50A Powerwall+ input: "With the factory-installed Neurio CT measuring solar circuits inside the Powerwall+ solar assembly, no field-installed CTs are required to measure Powerwall+ solar production."

I'm going to shut off that breaker and disconnect the CT myself later today (and let my installer know about their mistake. This was their first Powerwall+ installation). I'll report back afterwards, but I think that is absolutely the reason my production numbers are so high.

The Gateway is NOT being used as my main service panel, so I think it's correct to have the bonding strap installed.

Vines: THANK YOU for pointing out what we have all, including me, failed to notice: that the specs of Powerwall+ specify that the inverter can output 40A (9.6kW) off grid. Or this was so obvious and you all knew it, so no one felt it needed to be mentioned : ) Very cool feature of the Powerwall+.

The confirmation that the inverter can output 9.6 kW off-grid and the second CT seems to explain everything. Thank you for figuring that out, Vines.

I'll report back once my system gets PTO.

 

[wwhitney]

If you look a the inverter datasheet, it does specify that it can output 32A (7,6 kW) on grid, or 40A off grid.

That's what the PW+ data sheet says, 9.6 kVA full sun off-grid, but there's nothing saying that it can do that all from PV. That would be a PV plus ESS limit that the controller imposes due to the 50A breaker connection. The PV only limit may be lower and is unspecified on the spec sheet.

Cheers, Wayne

 

[jgleigh]

The Gateway is NOT being used as my main service panel, so I think it's correct to have the bonding strap installed.

It's normally removed unless it's the main service panel, but you should have the installer take a look. There might be something unique about how your system is wired.

 

[Vines]

Thank you everyone for chiming in! Lots to respond to, so I'll just do it all in this reply.

taphil: I don't have PTO yet so I can't run that test. In terms of mentioning overdrive, I'm referring to ericl at the very beginning of this thread saying that his Tesla Advisor said it was possible.

jgleigh: Damn, you're right! I just confirmed from page 55 of the Powerwall+ installation manual that they shouldn't have added that CT on the 50A Powerwall+ input: "With the factory-installed Neurio CT measuring solar circuits inside the Powerwall+ solar assembly, no field-installed CTs are required to measure Powerwall+ solar production."

I'm going to shut off that breaker and disconnect the CT myself later today (and let my installer know about their mistake. This was their first Powerwall+ installation). I'll report back afterwards, but I think that is absolutely the reason my production numbers are so high.

The Gateway is NOT being used as my main service panel, so I think it's correct to have the bonding strap installed.

Vines: THANK YOU for pointing out what we have all, including me, failed to notice: that the specs of Powerwall+ specify that the inverter can output 40A (9.6kW) off grid. Or this was so obvious and you all knew it, so no one felt it needed to be mentioned : ) Very cool feature of the Powerwall+.

The confirmation that the inverter can output 9.6 kW off-grid and the second CT seems to explain everything. Thank you for figuring that out, Vines.

I'll report back once my system gets PTO.

I have to say with that a caveat, that I don't install Powerwall+. It could also be a misconfigured CT and what the actual maximum PV that can be inverted isn't something I am 100% sure about.

So was your system double-counting the PV and it shows the output correctly now?

 

[Josh Gerritsen]

I have to say with that a caveat, that I don't install Powerwall+. It could also be a misconfigured CT and what the actual maximum PV that can be inverted isn't something I am 100% sure about.

So was your system double-counting the PV and it shows the output correctly now?

jgleigh: sorry, I said the opposite of what I meant. The gateway IS being used as the main service panel. And by that I mean my meter has no breaker switch, the gateway has one, as shown. And my main panel is being fed from the gateway.

Vines: I just removed the CT but it's cloudy and the sun is pretty low, so production is down to 1.4 kW. I'll check it on the next sunny day to see what it maxes out at.

 

[jgleigh]

jgleigh: sorry, I said the opposite of what I meant. The gateway IS being used as the main service panel. And by that I mean my meter has no breaker switch, the gateway has one, as shown. And my main panel is being fed from the gateway.

Good to hear. Yep, then that portion looks correct. Hopefully the extra CT removal fixes the other issues.

 

[Jeremy3292]

If you look a the inverter datasheet, it does specify that it can output 32A (7,6 kW) on grid, or 40A off grid. With your permission to export set to off, and your home loads utilizing more than 10 kW, I suspect the full 40A (9.6 kW) can be generated and used by your loads.

Perhaps a dumb question, but any thought as to why the Powerwall+ can use more power off-grid than on-grid?

 

[wwhitney]

Perhaps a dumb question, but any thought as to why the Powerwall+ can use more power off-grid than on-grid?

It's a "feature". The on-grid case is optimized for interconnection with a 200A main panel, which allows up to 32A of continuous inverter backfeed when using the 120% rule.

Cheers, Wayne

 

[Josh Gerritsen]

Well, that extra CT was certainly the cause of my high solar production numbers. I haven't seen the number go above 7.7 kW since. Perhaps it will happen later this spring.

 

[Vines]

Well, that extra CT was certainly the cause of my high solar production numbers. I haven't seen the number go above 7.7 kW since. Perhaps it will happen later this spring.

Glad you fixed it, and I learned something

 

[Jeremy3292]

I know it varies based on locale but for me here in SC with a south facing array, pvwatts says 11.2 kW panels on a 7.6 kW inverter (1.47 DC/AC ratio) produces a 1.3% loss, which is 210 kWh/year or 17.5 kWh/month. That really doesn't seem bad at all. It also says the clipping losses are heavily focused in the March-May timeframe when the temperature is in the 70's, which just so happens to be the ideal temperature that solar panel ratings are tested at. There aren't many clipping losses in the summer bc it's too hot which reduces the panels output already. There are also losses in the DC/AC conversion, cleanliness of the panels, etc. Solar inverters do best operating closer to max capacity at all times, so a higher DC/AC ratio means it'll ramp up earlier and stay ramped up later in the day also. So unless you're north of a 1.5 DC/AC ratio it seems to be fine or maybe even better than a 1.0 ratio.

 

[ericl]

I have not seen my inverter go higher than 7.7, however I am happy with the 1.5 ratio. I generated 16.2 MWH in 2022!!!!

68kw/day is the highest peak generation in June/July!

I also had lengthy discussions with my neighbor whom is a NASA electrical engineer that works with solar. 1.5 is good ratio. Don't be worried about the clipping. I was initially worried, but I don't think it really makes a huge difference at least here in Florida as long as its 1.5max because you need to factor in degradation of panels over time.