AC couple to existing grid tie system

[bmerrill]

Hello,
I've inherited an existing 18kW Tesla solar install at my current property. It's grid tied with excess power sold to the utility. I have been wanting to add backup battery power but find the total cost of adding a Tesla powerwall beyond ludicrous. I've been trying to read up on how/if it's possible to add my own and there have been some great posts on the forum that have both been enlightening and slightly over my head. I'm wondering if my basic understanding is correct/feasible or if I'm way off in the weeds of this actually working.

My existing install seems pretty straight forward. Two sets of panel strings feed into two Tesla inverters with the AC combined in parallel and then fed into my utility panel and the grid. My proposed high level design is to inject an AC coupled inverter like an EG4 12000XP into the system just after the PV/AC disconnect switch and before the utility panel. My understanding is that the AC output from the Tesla inverter AC connects to the "Smart Load" of the EG4 configured for AC Coupling and then the normal "LOAD" output then connects to the utility panel/grid. In the future I can add additional ground mount solar capacity directly via the 12000XP.

Existing PV/grid configuration:


Proposed PV/grid configuration:


Thank you for any insight

 

[DIYrich]

Replace 12000xp with 12kpv, and Move the grid to the 12kpv.

 

[bmerrill]

Ah, thank you. I think you're right with going with the 12kpv. I was struggling to understand the difference. The 12000xp has a single manual while the 12kpv has the information spread across several mini manuals. But I think it goes something along the lines of 12000xp == ac coupling with grid backup vs 12kpv == ac couple with grid tie. So I think for the 12kpv wiring I'm interested in the "12kPV with Supply Side Tap and Whole Home Backup" though I could do the Partial Home Backup and add a critical loads panel if I wanted to more incrementally scale up batteries over time.

Here's the updated high level diagram:

 

[DIYrich]

Move the Main Breaker Panel to the 12kPV.

The 12kpv can export. The 12000xp cannot. You want to be able to export your excess PV to the grid.

The Main Breaker Panel can have a Manual Transfer Switch, or Isolation device (if your main panel is compatible), so you can take the 12kpv off-line for servicing while powering the house from the grid.

Grid -> Transfer Switch Input 1
12kPV Load connection -> Transfer Switch Input 2
Transfer Switch Output -> Main Breaker Panel.

You will then want a disconnect switch between the Grid and the 12kPV.

Given that you have 15kW of panels, you may want the 18kpv. I think the 12kpv can deliver 8kw off battery, whereas the 18kpv can deliver 12kw. 15kW of solar implies a pretty big house. If you have a 5-ton AC (maybe even a 4-ton ac), you may want to consider the Gridboss/Flexboss 21 combo for the surge capability when the AC starts.

 

[1201]

@bmerrill you don't currently have a critical load panel which means if the grid is down you will not have any loads powered.

Currently you can feed from battery into loads at night which might be you goal, and if that's your goal then and ac retrofit might work just as well

 

[bmerrill]

Excellent information. I was definitely unsure how to isolate the inverter for maintenance. I think this wiring design is getting close to what I want to implement.

Given that you have 15kW of panels, you may want the 18kpv

You are absolutely right. My goal is total home backup and I don't think the 12kPV has enough capacity so I've switched over to the 18kPV for the 12K watt load. I have a 4 ton AC so I think it should be able to handle the startup surge based on the reviews and information I can find. The Gridboss/Flexboss combo is certainly a strange beast that I'm trying to read up on and understand. It certainly seems to have better surge so that alone might be worth the slight extra cost. And more capable of dynamically shedding heavy loads should demand outstrip supply or during a grid outage?

 

[dougbert]

The gridboss/flexboss combo is cheaper than an 18kpv + MTS + Disconnect, when you might want to add a another inverter, where you need combiner panels for AC in and AC out, watch more of the EG4 videos on them

The gridboss has a MTS in it already, thus you don't need the external MTS. Also has the disconnect

gridboss is $1800, a 200 amp transfer switch is $800 to $1200 by itself

flexboss is cheaper than the 18kpv

 

[bmerrill]

you don't currently have a critical load panel which means if the grid is down you will not have any loads powered.

Currently you can feed from battery into loads at night which might be you goal, and if that's your goal then and ac retrofit might work just as well

That is indeed my primary goal although I was hoping a secondary benefit would be to stay up during a grid outage, but maybe that's not possible without a critical loads panel. Anyway this is a completely new concept to me. Looks like there is a thread on the topic here. Reading up on it now...

 

[bmerrill]

The gridboss/flexboss combo is cheaper than an 18kpv + MTS + Disconnect, when you might want to add a another inverter, where you need combiner panels for AC in and AC out, watch more of the EG4 videos on them

The gridboss has a MTS in it already, thus you don't need the external MTS. Also has the disconnect

gridboss is $1800, a 200 amp transfer switch is $800 to $1200 by itself

flexboss is cheaper than the 18kpv

Oh, those are very good points. I hadn't researched the cost of the MTS yet. My initial reaction was that it made things more complicated. Now you are dealing with two components instead of a single package but from what I'm starting to grok, with the gridboss in particular, is that it's simplifying alot of the incidental wiring and components that you don't initially think about but are necessary for a traditional single inverter package.

 

[1201]

That is indeed my primary goal although I was hoping a secondary benefit would be to stay up during a grid outage, but maybe that's not possible without a critical loads panel. Anyway this is a completely new concept to me. Looks like there is a thread on the topic here. Reading up on it now...

If you want power during a grid outage you can either do whole house passthrough, which is generally not a good idea, or install a critical loads panel.

 

[Quattrohead]

You can make a simple non-critical loads panel with the grid boss, connect your heavy hitting devices to the smart ports of the grid boss and have them turned off when your battery gets to a certain state of charge or other programmable options.

 

[DIYrich]

The gridboss has a MTS in it already, thus you don't need the external MTS. Also has the disconnect

The MTS serves the same purpose with the Gridboss and the 18kpv - to take them out of the circuit for service.
If you don't want that ability, then there is no reason for an MTS and Disconnect with the 18kpv or the Gridboss.

 

[1201]

You can make a simple non-critical loads panel with the grid boss, connect your heavy hitting devices to the smart ports of the grid boss and have them turned off when your battery gets to a certain state of charge or other programmable options.

Or, buy two growatt sph10ks for the same price and have true whole home backup and better surge capacity

 

[rogerheflin]

Oh, those are very good points. I hadn't researched the cost of the MTS yet. My initial reaction was that it made things more complicated. Now you are dealing with two components instead of a single package but from what I'm starting to grok, with the gridboss in particular, is that it's simplifying alot of the incidental wiring and components that you don't initially think about but are necessary for a traditional single inverter package.

If you do a critical loads panel of say 60A (more than a 12kpv can fully power on batteries) then the transfer switch is much cheaper, and you do not need to move the main panel wiring around.