AC coupled recommendation?

[TexasSun]

Hi everyone,

I've currently got a 8kW system with enphase micro inverters. Works great, but being in Texas, with the big storm this past year, I wanted to upgrade to being off-grid capable. So, I've added 30 more panels with a Sol-ark 12k inverter and batteries. It's working great. Now, I've got a buddy with a matching 8kW enphase setup, and he's looking to do something similar with an AC coupled inverter, but doesn't want to spend the $7k for the Sol-ark12k.

So, he's looking at adding 5kW more solar panels with an AC coupled inverter and batteries. He's got the solar panels and batteries lined up, I'm just trying to determine the best inverter/charger to use. He really like the idea of AC coupling to be able to use his larger array when power goes off again, and dumping power into the batteries if he's not consuming that much at the time.

I like Schneider, but I don't think they're quite big enough. Outback is also not big enough. I see some interesting info on Growatt, but I don't know their history. And it seems their 'larger' systems are on paper-only, I've not seen them for sale. So, getting back to my question, what inverter/charger combos would you recommend to keep his costs down?
Thanks.

 

[Ampster]

I use an Outback Skybox but like any hybrid each has unique limits. One has to dive deep into the specs to really understand the capacity issue. There is a limit on the amount of AC coupled solar on most of those. Some of those can be stacked or paralleled so in those cases there is no limit on the total inverter capacity. In the case of a limit on the amount of AC coupled solar, a simple work around with micros is to simple shut off one breaker. This is only a constraint when the grid is down since that is the only scenerio where the GT inverter(s) are AC coupled.
It is hard to make a recommendation or give detailed advice without a lot more information. Based on the numbers provided in the original post, a Skybox has the capacity based on how my system is working. The Skybox is about $3k. AC coupling is a good way to leverage an existing Grid Tied system.

 

[sunshine_eggo]

Is the goal to run the entire house or power a critical loads panel? The latter is the far more common approach.

Broadly speaking, during peak sun, you have the full AC power of the PV array and the AC coupled inverter (20kW in your case), though that's obviously dependent on sunlight. Off sun, you're limited to your AC coupled inverter AND its power supply. Unless you have a metric assload of batteries, a 12kW inverter is severely compromised at night. In this context, $7K for the inverter is a tiny drop in the bucket compared to the $50K of batteries you need to make the system usable.

1. What is your battery bank capacity?
2. Did you conduct an energy audit when you designed your system?
3. Has he?

My concern is that a lot of dollars are being thrown at something with insufficient planning.

 

[Ampster]

I agree with @sunshine_eggo that planning is everything to avoid wasting money. It is a matter of balancing the amount of AC coupled GT solar and DC coupled hybrid inverter capacity. Designing around the loads that would be needed to be served in a grid down scenerio is essential. Battery capacity can effect the outcome since the batteries are a buffer to help modulate the AC coupled micros.
Unless one is contemplating very expensive batteries it can be done for a lot less than $50k. For example, my DIY pack of 42kWh LFP batteries cost about $6,500 including an expensive BMS and other components.

 

[sunshine_eggo]

I agree with @sunshine_eggo that planning is everything to avoid wasting money. It is a matter of balancing the amount of AC coupled GT solar and DC coupled hybrid inverter capacity. Designing around the loads that would be needed to be served in a grid down scenerio is essential. Battery capacity can effect the outcome since the batteries are a buffer to help modulate the AC coupled micros.
Unless one is contemplating very expensive batteries it can be done for a lot less than $50k. For example, my DIY pack of 42kWh LFP batteries cost about $6,500 including an expensive BMS and other components.

True, but we're also talking about what it's going to take to get days of whole house power with poor solar. 42kWh is probably the very low end, and 100kWh is probably the starting point. The conditions of the TX storm were NOT optimized for solar. If they're planning for a week-long power outage in February sun, they need a lot of battery backup to power their whole house. Imagine if they're running heat pumps - 3-4kW needed mostly at night.

Doubt they'll wait 8-26 weeks for LFP cells from China.

Four of these, expensive compared to DIY, but are ~40% less than BattleBorn:

48V Rhino KIT - LFP - 552Ah - 28kWh - BigBattery.com

THE WORLD’S THINNEST WALL-MOUNTED BATTERY! This battery now comes in a Kit for applications such as off-grid solar, industrial, and more! It has a power density of 28kWh of capacity and is also the strongest wall-mounted battery on the market today.

bigbattery.com

will set you back over $50K.

 

[Ampster]

Big Battery has a poor reputation. There is Signature Solar somewhere in Texas that has a modular drop in rack mount battery that is $300 per kWh so that would be $30.000 for 100kWh. Your friend could pick them up to save freight and derisk the transaction in terms of stock on hand.

No storm in any part of the world is optimized for solar. However solar can be optimized for cold weather and snow in an emergency. Long after the storm was gone, my understanding is there were clear skies but very cold temperaturesand no power because the natural gas distribution system was frozen. It all depends on how your friend wants to manage that risk. Removing some snow on panels might be worth saving some money on batteries. Also loads can be managed during a grid down situation. I am not disagreeing with what you are saying. I am just providing alternatives to other readers who may want to see the alternatives to the system you are suggesting.

 

[sunshine_eggo]

But.. but... they have award winning customer service!!!

Even so, if $30K batteries is on the table, again, saving a couple $K on an inverter is likely the wrong place to look.

 

[DCPower]

Installing enough battery to go full tilt boogie through days of overcast and no grid energy is never cost effective. Off grid systems design starts with load evaluation. Then battery. Last, the PV battery charger. It’s a shame so many people were sold Enphase AC module solar - even enphase has to juggle chainsaws to add battery to their own systems. Honestly, a cheap propane generator would be the easiest way to recover from buying grid tied rooftop solar. ?

 

[Ampster]

saving a couple $K on an inverter is likely the wrong place to look

Assuming that they need 100kWh battery is the wrong place to start. As @DCPower mentioned a load analysis would be the best way to make an assumption about battery size.

 

[sunshine_eggo]

Assuming that they need 100kWh battery is the wrong place to start. As @DCPower mentioned a load analysis would be the best way to make an assumption about battery size.

My first response sent at the exact same time as yours indicated the need for an energy audit.

The OP appears to be of the belief that a 12kW unit is needed to provide backup power for the 2021 TX freeze. If that's the thinking, then his loads require a 12kW unit, and 100kWh of battery backup is not at all unreasonable given that scenario.

I'm of the belief that the OP purchased an overpriced/overkill unit that will not meet the intended goal likely due to a lack of an energy audit, insufficient batteries and poor system design.

 

[Ampster]

The OP appears to be of the belief that a 12kW unit is needed to provide backup power for the 2021 TX freeze. If that's the thinking, then his loads require a 12kW unit, and 100kWh of battery backup is not at all unreasonable given that scenario.

I don't know. The OP did purchase a 12kW inverter but did not mention the size of the battery he purchased, He gave no information about his friend and what size pack was needed, Rereading the thread, I realize you made the assumption about a 100 kWh battery and mentioned $50,000. We have not heard from the OP since his initial post.

 

[TexasSun]

I do appreciate the replies. My apologies for not replying over the weekend, I was out of town with snail-speed internet.

OK, don't worry about the batteries in either setup. We each have about 20kW-hrs of batteries at our disposal. With more available when we want it. Call it a blessing we each enjoy.

The key part of my question was inverter/chargers which got lost in the rabbit-trails from the weekend. Keep in mind, I'm an electrical engineer and fully understand the trade-offs/balance issues.

The key factor is that the unit needs to be able to absorb 8kW from the old enphase inverters and dump that into the batteries. And it needs to absorb about 5kW from the new panels, again dumping that into the batteries. At 48v, we're talking 160 amps from the old inverters and 100 amps more from the new panels coming through the new inverter/charger. Total 260 amps(worst case, no load, full sun... unlikely but I don't want to toast things). I don't know if some units can just ignore its 5kW of solar if it's maxed out sucking power from the old 8kW setup?

So the options I'm seeing (and looking for more):
1. Sol-Ark 12k. It's what I got & it's working great, but he doesn't want to drop $7k on it. (well dang, I just double checked the specs, and it is 185 amp max DC output... need to scratch my head a little on this, but with my wife only knowing how to turn on a light and not knowing how to turn off a light switch, we'll never be drawing 0 load)
2. Sigineer M12040D. It only puts out 100 amps to the batteries, but I've read a number of good review of the company on this unit. I don't know if that unit can marry to an additional charger to get more DC amps output?

I've seen numerous smaller inverters that can stack, but I'm not sure they can handle the AC coupling and higher current needs. Schneider & Outback are examples. I'm seeing a lot of people talk about Growatt, but while I read about their bigger units, I have not seen any actually 'for sale'.
Thanks

 

[Supervstech]

My first response sent at the exact same time as yours indicated the need for an energy audit.

The OP appears to be of the belief that a 12kW unit is needed to provide backup power for the 2021 TX freeze. If that's the thinking, then his loads require a 12kW unit, and 100kWh of battery backup is not at all unreasonable given that scenario.

I'm of the belief that the OP purchased an overpriced/overkill unit that will not meet the intended goal likely due to a lack of an energy audit, insufficient batteries and poor system design.

I’m thinking, in a grid down gas lines down… winter weather situation… 100Ah isn’t unreasonable… the amount of energy electric resistive heaters use is STAGGERING…
Then again, the odds his will happen again is low. Likely whatever took down the gas mains has been corrected. I’m certain MANY homeowners in the area purchased standby generators after the fact…

 

[sunshine_eggo]

I made a couple errors and deleted my post.

 

[Ampster]

The Outback Skybox is reasonably priced at about $3k and is only limited by its 60Amp AC breaker which is 14kWs. balancing the DC coupled and AC coupled solar might be an issue when the grid is down but 20kWhs of batteries to provide the necessary buffer is workable. I am using a Skybox with about 6kW of AC micro inverters but have not tested it yet in a grid down because I am in the process of reconfiguring my battery pack. Depending on how you configure the Outback Radian it can be acquired for less. The Skybox is an All in One solution but the Radian needs separate charge controller and does not have the integrated automatic transfer switch like the Skybox does.

 

[TexasSun]

Ampster,

I had looked at the Outback Skybox, and just spent another hour reading about it. 5kW solar input is sufficient. Price is fair. But I have yet to find how much power it can dump into the batteries! It does state that it can work with 7.6kW of external solar/inverters that are UL 1741 compliant (my M215's are), and that's close enough to the 8kW system in place. But I really would like to know the number of amps that it can dump into a 48v battery. Do you have a link that has more info?

And Supervstech, heat really isn't an issue. My house is so well insulated that it needs to drop below the 30's outside before I even think about some heat (and I have a small fireplace). My buddy's place has a big old fireplace that could heat a school. A hurricane is a more likely and more strenuous scenario for my buddy & I. Running the AC, the well pump, etc.

Thanks

 

[Ampster]

. Do you have a link that has more info?

The battery breaker is rated at 175 Amps.
I will look for that info. My Internet just went down and my Andoid is slow.
I think it can charge at least at 80 Amps (48 volts nominal)
outbackpower.com
The M215 are UL1741 compliant which means they can AC couple. You may want to check if they are UL1741SA compliant which means they could modulate. Otherwise when grid is down they will be on until batteries are full and continue if there are loads. That is not a limitation of the Skybox, which is UL1741SA compliant. I have Enphase IQ7 which can modulate.

 

[Hedges]

Another AC coupling solution is SMA Sunny Island. MSRP will be more than you want to pay (for 2 of them!) but liquidation deals are still out there.
Each one will pull down 5kW continuous to a battery, so 10kW for two. How much you want to charge will depend on size/chemistry of battery.
I have 20kWh of AGM, and charge it at 4kW. AC coupled PV much larger than that. It is grid-backup, so if grid down most power runs the house directly, and excess production is curtailed.

 

[TexasSun]

Ampster, good point on the UL1741 vs UL1741SA. That cuts the 'other' solar support almost in half. And 80 amps at 48v is only 4kW, so that becomes woefully insufficient. He'd need two of them, which puts him back in the $6k+ range.

Hedges, I've seen a few discussions questioning if SMA really works in these modes. And, as you said two are necessary, which again is $7k+.

 

[Hedges]

Sunny Island MSRP is > $5k, retail maybe a bit < $4k
still some liquidation sales due to a large customer bankruptcy.
There are cheaper inverters. This one has good power handling, temperature management, and people say it has very good algorithm for lead-acid (longer life). For Lithium, it depends on a compatible BMS to request charging parameters.

The following ad says "new" but indicates used in the text. 2 available, $2500 each

SMA Sunny Island Off-Grid Inverter (SI6048-US-10) for sale online | eBay

Find many great new & used options and get the best deals for SMA Sunny Island Off-Grid Inverter (SI6048-US-10) at the best online prices at eBay! Free shipping for many products!

www.ebay.com

You could use just one with an auto-transformer for 120/240V conversion. Grid-backup, if you didn't have a transformer on both input and output, all the power would come from one phase. Having two is better, and doubles pass-through power (56A per phase) and power handling.



Sunny Island works great passing through grid power, transferring to battery power, operating with GT PV inverters managed by frequency shift.
I only use it with Sunny Boy, don't have experience with Enphase or other inverters.
If the inverter is only UL-1741 (like my Sunny Boy in grid-tie mode), frequency cycles up and down, knocking Sunny Boy offline repeatedly, not the operation you want.
With an inverter following just just UL-1741SA but also the optional frequency-watts, frequency hovers > 60 Hz, regulating output to match load requirements.