Adding battery backup to existing grid-tie system

[Awsmits]

I’m not new to solar systems, having built my own off-grid system with DIY EVE 280Ah LiFePO4 cells, a Growatt 3000W all-in-one inverter, and 2000W of solar panels. I understand grid-tie systems, but have not worked on them. I’ve learned a lot from this forum and Will’s videos.

So I thought it wise to reach out to this community when a family member in California who has a Sunrun-installed residential grid-tie system asked me to help him design and install a battery backup to his system, given the increasing threat of power outages in that state.

The specs on his system are:
- 30 panels, 350W each for a total capacity of 10.5 KW
- Solar feed is a single feed into a Solaredge HD Wave inverter (I believe it’s the SE 7600 model). Input labels on inverter state max input of 450V and 32A. Output of inverter is 240V AC.
- The AC output of the inverter is passed through an AC shutoff box, before being routed to the main circuit breaker box, where it is coupled to the panel through a pair of 40A breakers.
- We have confirmed that shutting off the grid input shuts down the inverter AC output, as it should.

I’ve read enough to know that the battery backup system (inverter and batteries) can be set up via AC-coupling or DC-coupling. Based on my experience, I think DC-coupling would be easier in his system, but requires a largish inverter that can handle the 450V input, of which there are several. Ideally, during a blackout with the grid down, I’d like to use the solar panel DC power run through an off-grid inverter to run essential loads and charge the batteries during the day, then rely on batteries for the essential loads at night. Essential loads can be 120V only, but 120/240V are preferable.

What do you folks think? Is DC-coupled practical here, and how would I wire it in? Should I attempt a schematic for review?

Thanks!

 

[400bird]

Solar Edge uses panel level optimizers.
If you are planning to remove the Solar Edge inverter (to install the Solar Charge Controller), you'll either need to figure out how to send an enable signal to the optimizers or remove/replace them all.

Without the enable signal from the inverter, the optimizers limit output to 1 volt each, so you'll have to do something with them.

 

[Awsmits]

That’s a major concern you bring up…didn’t know that. No, I was not planning on replacing the grid Solaredge inverter as that’s needed when the grid is on. I was thinking of re-routing the panel DC power (via a manual switch) to a second inverter that can then power the critical loads and charge batteries. It could be a second Solaredge inverter, or similar.

 

[400bird]

If you want to leave the Solar Edge inverter in place and add a second (battery) inverter, that would be well accomplished by AC coupling.

What is the goal? Just back up power? Any desire to timeshift current production? Fully automatic back up or do they want to manually go out and throw some switches to disconnect from the grid and power up the battery inverter?
What's their back up load usage?

I'd be looking at something like an Outback Radian, SMA, or Schneider XW 6848 (I have the XW, there are a couple notes on usage if they go this way), I believe Sol-Ark would work too.

I would recommend a grid forming inverter with integrated transfer relays.

 

[Awsmits]

The goal is backup power only, manual switching is fine. Nothing special or automatic. Backup load usage would be in the 3-4kW range at most, by selective use of loads.

Talk to me about AC coupling of two inverters…how is this done (wired), does the second (off-grid) inverter need to be ”compatible” with the grid inverter (Solar Edge), and I assume one sets the parameters in the off-grid inverter to decide how to charge the batteries and essential loads?

 

[Ampster]

Talk to me about AC coupling of two inverters…how is this done (wired), does the second (off-grid) inverter need to be ”compatible” with the grid inverter (Solar Edge), and I assume one sets the parameters in the off-grid inverter to decide how to charge the batteries and essential loads?

I use an Outback Skybox AC coupled to Enphase micros..Previously it was AC coupled to a SolarEdge HD Wave so I know they would work together. It has several modes that allow me to ride through high rate periods so I use it not only for backup but for load shifting..
The wiring would require a critical loads panel powered by the grid forming inverter. The existing solar would also be wired to that panel. The Skybox is limited to 7.5 kW of AC coupled solar based on inverter capacity so you are right at that limit. All the earlier recommendations would also work provided you verify their limit on AC coupling.

 

[Awsmits]

So, I think what I’m hearing is SolarEdge has made it impossible to DC couple, unless it’s connected to their Energy Bank system? Is that right? Too bad, as that would be most efficient.

The Outback, SMA, Schneider and Sol-Arc inverters for AC coupling are very expensive. Are there any other more affordable inverters that would be compatible with the SolarEdge HD Wave? How about Victron?

 

[400bird]

So, I think what I’m hearing is SolarEdge has made it impossible to DC couple, unless it’s connected to their Energy Bank system? Is that right? Too bad, as that would be most efficient.

Solar Edge makes grid tie inverters, not charge controllers. If you want to DC couple, you'd need to remove the grid tie inverter. (Or buy one of the specific inverters they make specifically for battery back up)
AC coupling is more efficient if you use more power directly when the sun is shining.
DC coupling is more efficient for power used when PV isn't producing.

The Outback, SMA, Schneider and Sol-Arc inverters for AC coupling are very expensive. Are there any other more affordable inverters that would be compatible with the SolarEdge HD Wave? How about Victron?

Victron is in the same boat, cost wise. It will work, and they have a great rep, but they don't make a split phase 120/240 inverter, so you'd need two or an auto transformer. The deal breaker for me is that they aren't UL listed and can't be installed with a permit in my area and most of the US.

There may be a Growatt, EG4, or some similar inverter that functions similarly, but I don't have the expertise to tell you about any inverter at that level.

 

[MurphyGuy]

I use AC Coupling with a couple of Grid-Tied SMA Sunny Boy's feeding the grid, and two SMA Sunny Islands for our off-grid system.

We keep our off-grid system completely isolated when not in use. Not even a neutral or ground is connected. When the grid goes down, I plug a 4 gauge patch cord in, flip the interlocked breaker that isolates the house from the grid, and the house gets power. A few minutes later, the grid tied system kicks in and starts making power to run the house and charge batteries.

Its easy, fast, and I don't have any extra components like charge controllers, fuses, combiner boxes, etc.

Here's a picture of the off grid system..

 

[Ironman482]

I want to do a similar battery backup system on my existing 9kw system my panels have micro inverters on them and they feed to a emphase junction box , I called the installer and they only use emphase batteries, I don't want to use the enphase batteries because from what I've read they are not designed for backup power, was wanting to use reony 200ah AGM batteries, 4 12volt for 48 volt , none of the solar contractors around my area will even give me a estimate on a batt backup system , I'm fairly good at electrical wiring ,but don't feel confident enough to DIY ,and worried that I might damage my existing system , is there any litature or video on wiring something like this outthere?

 

[Hedges]

What model microinverters? If they do UL-1741-SA frequency-watts, then should be able to AC couple to battery inverters designed for that.

Here's a packaged assembly made by DC solar:

Complete Solar Generator Cabinet w/ QTY 2 SMA Sunny Island 6048 w/ Warranty | eBay

Find many great new & used options and get the best deals for Complete Solar Generator Cabinet w/ QTY 2 SMA Sunny Island 6048 w/ Warranty at the best online prices at eBay! Free shipping for many products!

www.ebay.com

I used SunXtender AGM batteries:

Sun Xtender Solar Batteries - Photovoltaic Battery

The Sun Xtender Battery line consists of specialty lead-acid solar, photovoltaic batteries, and renewable energy storage batteries manufactured by Concorde Battery Corporation.

sunxtender.com

 

[Ironman482]

The panels are Trina 295,s not sure on the inverters, I can contact the sunpro and find out , how many yrs do you expect to get with the AGM batteries, I just can't justify the cost of lithium batteries.

 

[Ironman482]

The panels are Trina 295,s not sure on the inverters, I can contact the sunpro and find out , how many yrs do you expect to get with the AGM batteries, I just can't justify the cost of lithium batteries.

 

[Hedges]

Per technical data sheet I expect 10 years float life or 650 cycles to 70% DoD.
I have a 14 year old set (that were occasionally charged, not kept at float) which still function but are about 40% of original capacity.

2.5 years ago, Lithium was 5x the cost of AGM, 5x the cycle life. Same cost per kWh of cycling, if cycled nightly.
AGM prices same or a bit higher today, $0.50/kWh of cycle life. FLA about $0.25/kWh (and possibly 20 year life at 15% DoD.)
Lithium prices have dropped, similar purchase price per kWh of capacity as AGM, as low as $0.05/kWh over claimed 6000 cycle life (16 years nightly cycling.)

I believe in absolutely minimizing size of battery bank, barely one night's consumption, at least for backup system. Full off grid, may want more unless you have large PV array and can avoid using much power during heavy clouds.

 

[Hedges]

LiFePO4 server rack batteries for about $1500, 100 Ah 48V, look attractive. Same price as quality AGM and 5x to 10x the cycle life Biggest issue appears to be max current draw, so OK for small inverter, otherwise need a stack of batteries.