10kW solar with single Powerwall setup question

[keeperlink]

Hi,
I'm planning to install 10kW(AC) solar with one Powerwall.
My installer suggests to put two 5kW inverters and one of the inverter will be on automatic disconnect that will be OFF during power outage. The reasoning is that Powerwall cannot consume more than 5kW during charging cycle.
Is it the right approach? In that configuration I'm loosing half of solar production even in low solar or high house loads (I have Tesla EV as well)
Can it be rather configured by slightly different AC frequency cut off on each inverter?
Thanks

 

[tomuo]

You may need to sign a waiver for the design, but having both inverters on the load side (backed up) is possible, but is not recommended by Tesla.
There's only one frequency when you're island-ed, so worst case is that both inverters are putting out too much power for the powerwall, and the whole system shuts down and won't power up again until the grid comes back.
In practice, the gateway software manages this fine, but it's a scenario that is outside of their recommended envelope.

So ask for both on the powerwall side, saying you're OK with the consequences of that possibility of a grid outage in mid summer taking the system offline for the rest of the day. They probably won't understand that part though.

 

[tomuo]

It also leaves it open for you to add another battery in the future without having to invasively reconfigure the existing wiring.

 

[jjrandorin]

Hi,
I'm planning to install 10kW(AC) solar with one Powerwall.
My installer suggests to put two 5kW inverters and one of the inverter will be on automatic disconnect that will be OFF during power outage. The reasoning is that Powerwall cannot consume more than 5kW during charging cycle.
Is it the right approach? In that configuration I'm loosing half of solar production even in low solar or high house loads (I have Tesla EV as well)
Can it be rather configured by slightly different AC frequency cut off on each inverter?
Thanks

Its my opinion that no one who can actually consume 10kW of solar should have only 1 Powerwall, unless the goal is to only power "lights and a fridge" during a power outage. If thats your goal (lights and a fridge) then either a generator is cheaper, or, having half of your PV off doesnt matter.

The installer is trying to prevent the call back of "I dont understand why my solar didnt work during a power outage, and neither did the battery, I couldnt turn anything at all on" that could happen if your system generates enough power during an outage to knock the PW offline.

You might be able to sign a waiver like @tomuo says that in effect says "I am accepting responsibility for this possibly not working during an outage" but I wouldnt want that for myself. Too easy to get stuck with "any" issue being thrown back in your face with "here is the waiver you signed" if push comes to shove.

First, determine what your goal is, why do you only want 1 powerwall with 10kW of solar, when tesla recommends no more than 7kw of PV per powerwall (with the expectation that 7kw of PV likely wont generate more than 5kW peak at any one time).

if its "lights and a fridge" then you shouldnt have any concern about the setup of 1/2 your solar being off. If its "oh yeah and I want to charge my car" then examine why you are only wanting 1 powerwall (you likely want 2 with 10kW of PV capacity).

 

[keeperlink]

Thank you for you replies tomuo and jjrandorin.
Right, that's the idea. I planned to add a second Powerwall later when battery prices go down. I didn't realize the issue I described above until recently when I got electrical drawings. Now I'd be willing to add a second Powerwall right now, but installer says it will require to re-submit applications for Tesla and power company approvals that will delay project for another month or so. Also I afraid the installer will take the opportunity to recalculate whole project cost with new inflated prices

 

[jjrandorin]

Thank you for you replies tomuo and jjrandorin.
Right, that's the idea. I planned to add a second Powerwall later when battery prices go down. I didn't realize the issue I described above until recently when I got electrical drawings. Now I'd be willing to add a second Powerwall right now, but installer says it will require to re-submit applications for Tesla and power company approvals that will delay project for another month or so. Also I afraid the installer will take the opportunity to recalculate whole project cost with new inflated prices

A Big (huge) mistake people make in doing this stuff, at least in my opinion, is thinking that there is some way that they can "set themselves up to add more later easily when XXXX happens".

That isnt going to happen. You will NOT get an install where something is "easy" to add later, nor will anyone want to touch anything to "easily" add it later even if you manage to get it configured as such from a hardware / install point of view.

No one is adding more panels onto an existing install unless they specifically are the ones that installed it (and likely not really even then, unless it was like a year ago or something). No one is "easily" adding on an additional battery for a super reduced install charge vs "the regular" install charges.

If you are thinking "Oh I will get one battery now, and get one later when they are cheaper, and it will be easy to just add that on there"... unless you are capable of doing the physical install yourself, that isnt happening (and even if you are capable of doing it yourself, you still would need someone to sign off on it and basically accept responsibility for it, which will be a fight at best.

Get what you want now, and plan on any additions being " a new system tied into my current system, with new system installation costs and timeline", and you wont be disappointed. If you are considering getting 2 powerwalls now, get them. Better to wait an extra month now, then try to do it later, find out that the single powerwall add on is going to cost you 10k, and take 2-4 months for someone to do it because they are too busy doing PV + battery installs to do a low hanging fruit job like that.

 

[miimura]

If you think you eventually want the second one, just do it now. The price won't necessarily go down and the hassle just isn't worth it.

 

[keeperlink]

Thank you guys. Good point. I will follow your advice

 

[tomuo]

Yes, get the quote for the system with 2 batteries. My timeframe for hoping battery prices go down is 5-10 years, not 1-2.

 

[Krash]

Battery prices are likely not going down. Everything is going up.

That said, I have seen a single older Powerwall2 on 7.5kW of panels knowing that the homeowner will have to shut down one third solar array mid day if the grid is down to avoid peak power having nowhere to go. Either that or schedule car charging during that peak.

Not ideal, but cheap. Also that setup was wired with 50A wire to the PowerWall for some future proofing.

 

[keeperlink]

Agree.
Another issue my installer just pointed out. I was planning to put 18kW panels on 10kW inverter. But inverter is rated max 15.5kW DC. My roof is not ideal - facing east/west. I did calculations on pvwatts - my setup will peak at 14.8kW DC. Will there be complications with a warranty or should I reduce panels to meet the inverter rating?

 

[jjrandorin]

I'm planning to install 10kW(AC) solar with one Powerwall.

Another issue my installer just pointed out. I was planning to put 18kW panels on 10kW inverter.

Typically, I believe the amount of PV is quoted by the panel, so your install is actually "18kW PV" not "10kW of PV", even if you only have a 10kW inverter.

You typically want a ratio of panels to inverter, what that ratio is depends on the home but thats what you pay your installer / solar company to help you design. That ratio sounds out of line to me, but I am not a PV designer.