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Outback Radian GS8048A in AC coupling mode - max GDI question

SparkyJJO

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I have 7.92 kW (by label) of grid tie using Chilicon microinverters. I'm in the process of adding AC coupled battery backup for a few reasons (want backup in case of grid failure from a storm, or grid instability due to geopolitical stupidity, or whatever). The upcoming Outback Mojave looked like the perfect inverter for this, nice updated interface, capable of up to 8kW of GDI output.

Then I got to thinking about it more - the Mojave is more limited as storage only and cannot accept any kind of direct PV input. If we ever moved (we've toyed with the idea off and on), I'd have to get more grid tie inverters to make it work at the new location. Maybe not a big deal, maybe it would be. If I got the Radian, I could just take it with us (and the battery pack), and "simply" add Outback charge controller(s) and panels at the new location and be up and running. Downsides - a clunkier interface (minor), and it is rated at a max of 7.6kW of grid tie. I have 7.9kW - what to do about the extra 300W above that? I could split it up as I have dual lines coming down from the roof, and have half before Radian and half after, but then that really reduces the solar I could use in a grid outage, when I could really use the most solar I can get.

What's the pro's thoughts on these? What would you guys do? If the Radian, what to do about the excess 300W?

Also, a side note about the Mojave - Outback told me via email it is a LF inverter, but looking at the spec sheets, the Mojave has a peak power of 10kVA, while the Radian is 17kVA for 100ms, 12kVA for 5 seconds, or 9kVA for 30 minutes. Maybe they are both LF, but the Radian is just more robust? Not sure how much it matters as the biggest inductive load I have is AC, and I'd only run that in a grid-down situation on a hot sunny day when I have plenty of excess power (and even then I might not use it depending on other loads, my AC isn't all that efficient).
 
Mojave no-load idle is spec'd at 100 watts compared to Radian spec of 30 watts.

Weight is about the same, Mojave is LF inverter.

mojave.png
Mohave has slightly higher AC pass-through current.

Don't forget your battery sizing for AC coupling. Battery must be sized big enough to absorb momentary back surge PV over-production unless you have a load dump system.

Only significant difference I see is the high idle consumption of Mohave. Mohave has more appearance flare, Radian looks like a T-38 tank.

My guess is Mojave is probably designed to give better Outback profit margin over Radian.

I would not want to be first users of a new inverter model before bugs worked out.
 
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Thanks for the input. My battery pack is going to be larger than the complete Mojave "kit" they're selling, so no issues there.

I don't care about appearance so much - I like the styling of either one in their own way anyway - just the interface I know from reports is a bit clunky on the Radian, with options still available in the menus that do not apply causing clutter/confusion. I know I can work through that however, just an updated, more refined interface would be nice.

I was also a bit concerned about being an early adopter. Could be fine if they vetted it out well, but far too often these days companies release products with "good enough" then fix stuff via software updates later, whenever they get to it.

Biggest thing I'm not sure about on the Radian is what to do about my 300W above the AC coupling spec. (And interestingly enough, my 7.9 kW array just hit a brief peak of 9 kW output about an hour ago. The Radian's AC coupling information indicates 7.6kW max coupled "by the label" so it must have some level of wiggle room.
 
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On GT inverter AC coupling to hybrid inverter output there is no back flow battery push current regulation other than inverter DC input overvoltage shutdown.

It is always safer if you have a load dump, like a hot water heater, when back feed to hybrid inverter gets too great.

With grid down situation, freq shifting to control GT PV inverters is not fast enough when you have high PV production, heavy house loads, then a large house load is shut off creating a sudden overproduction that must be dealt with by hybrid inverter and batteries to suck it up.

You can always split up the GT PV with some put on direct grid side and some on hybrid inverter output. Just lose the direct grid PV when grid is down.

I have also seen a high battery voltage trigger that pulls the plug on GT inverter at a maximum battery voltage, but you still need to keep max PV power under the power capability of hybrid inverter.
 
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Oh. I did not realize the Radian had no regulation that way. I made the assumption that it handled the charging like normal either way. Hm, is this hard on the batteries then if there is no charge profile being followed? Well, I guess maybe that only occurs when in a grid down situation, since when grid is up it just pushes all extra back to the grid and can charge and maintain the batteries as normal?

Maybe the Mojave is better in this use case as it is designed specifically for this purpose?
 
Oh. I did not realize the Radian had no regulation that way. I made the assumption that it handled the charging like normal either way. Hm, is this hard on the batteries then if there is no charge profile being followed? Well, I guess maybe that only occurs when in a grid down situation, since when grid is up it just pushes all extra back to the grid and can charge and maintain the batteries as normal?

Maybe the Mojave is better in this use case as it is designed specifically for this purpose?
Without grid, no AC coupling hybrid inverter without a load dump has battery back feed regulation. Power generated has to go somewhere.

SolArk (HF) hybrid inverter has load dump control capability (you supply the load dump). I would never run a HF hybrid inverter without load dump with AC coupling. They are too delicate in back feed high freq HV DC to battery DC converter. Without load dump, they recommend PV GT inverter feed in through Gen input port so they can open relay and cut off GT feed if things get out of hand.
 
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At least the Outbacks are LF. I prefer the behavior of LF anyway.

I'll have to see if the Radian has any load dump configuration ability. For some reason I didn't even think to look into that.

*EDIT* So yes, the Radian has an AUX dry contact set that can be used for load diversion, up to 10A at 240VAC. Thing is, the manual PDF on their website is from a few years ago and does not cover AC coupling at all, so I have no idea if all the details are accurate. Outback Power really needs to update their website and links to the most recent versions of everything. Their Radian AC coupling link, for example, goes to the old 2019 version that lists 6kW AC coupled max, while if I dig around for a while I find a newer version from 2020 that lists the updated AC coupling of 7.6kW.
 
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So I'm more heavily leaning toward the Radian now because 1) it is available now and 2) it may provide more long term flexibility. Sure, the updated, evidently more intuitive interface of the Mojave would be nice, but I can work around it. Out of the box support for my array would be great too. But I just checked in on the pricing and availability and Outback is too focused on their complete ESS solution (priced at $16k!) to even give an idea of rough price of the inverter only.

I just need to figure something out about the max AC coupling amount and that 300W of excess I have.

I've considered a relay setup that disconnects the AC from a single microinverter when the Radian would switch into inverting mode, using the AUX contacts. I'd have to get some wiring up to the microinverter and all that could be loads of fun, but I think it should work, in theory.
 
I have a GS4048 AC-couple to 24 Enphase inverters (7kw) and 3 APsystems QS1 quad inverters (3kw) and DC-coupled with a Midnite 200 SCC on 5kw of solar tied to that. With a 35kwh battery.

To AC-couple you have to do a firmware update which is easy. I have ran all 10kw of solar through the unit and it works. I normally have the 7kw of Enphase on the grid side of the GS4048 and the 3kw of APsystems on the output of the GS4048(AC-coupled), but if the grid goes down for an extended time and batteries are low I have a jumper that can send all 10kw of solar to the GS4048 AC-coupled....

Being that I am DC-coupled I have to change the battery charge settings in the GS4048 to charge off the invert/AC-coupled solar at a higher rate because that is adjustable if you want to charge off AC-coupled. Normally I send the 7kw to grid(Net Metering) and the rest powers the house at night with the batteries.

The outback display shows everything that can be connected to it even if you don't have it so that is confusing at first. But just use the menus of what is connected and it is easy.

I am guessing that my AC-couple works over the spec because the Enphase come on nice and feather up and down in steps as the AC-coupling control functions....The APsystems inverters are either off or on like a light switch so I don't like them for AC-coupling but they work.

The GS4048 has no MPPT solar inputs....but you have micros so you are good.
 
Good to know. My Chilicon inverters are supposed to be nice and smooth as well, and if I'm reading the documentation right, there may be a way to limit the max power output manually if necessary to effectively make it a 7.6kW array and call it a day.

Then it dawned on me, after thinking about the array of panels on my roof and then looking at the array drawings again.

I have 21 REC 400W panels. 20 of them are connected to 10 CP-720 inverters, each of which does 720W continuous output. The last panel is connected to a CP-250 inverter, which can do 289 continuous. That's about 7.5kW array. D'oh. All this angst over nothing lol.

In case you're wondering how I screwed that up, I originally was quoted 11 CP-720 inverters and 22 panels, but because of some roof venting that was in the way they had to drop one panel, and thus changed the inverter to match it.
 
My guess on max ac-couple solar is because the amount of power that has to go backwards through the unit when grid tied and the relay can't handle it....
 
My guess on max ac-couple solar is because the amount of power that has to go backwards through the unit when grid tied and the relay can't handle it....
That is a limitation when grid tied active but AC input connect/pass-through relay is usually much higher amperage capability than inverter power capability to suck up back feed push to battery when grid is down.

On Radian inverter, AC pass-through relay is rated for 50 amps while inverter AC is capable of 33 amps.
 
That is a limitation when grid tied active but AC input connect/pass-through relay is usually much higher amperage capability than inverter power capability to suck up back feed push to battery when grid is down.

On Radian inverter, AC pass-through relay is rated for 50 amps while inverter AC is capable of 33 amps.
Plus with sell to grid on it puts you over; still looks like the relay could the problem?
 
No, 7.6kW of grid tie is only 32A - far below the 50A limit. I believe it has to do with being able to absorb high output momentarily if loads drop suddenly and it has to do freq shift to curb the grid tie output. That power getting shoved by the grid ties has to go somewhere.
 
No, 7.6kW of grid tie is only 32A - far below the 50A limit. I believe it has to do with being able to absorb high output momentarily if loads drop suddenly and it has to do freq shift to curb the grid tie output. That power getting shoved by the grid ties has to go somewhere.
I am guessing you don't have this inverter and are just guessing. I have one and am still guessing.... :)

Anyway when the AC-coupled solar is making full power yes it is putting ~32amps out, but a lot of these systems are setup as mine is with both AC and DC coupled. Most DC-coupled setups use a solar charge controller to charge the batteries and when the batteries get above a certain voltage then a thing called Grid Ties Sell turns on the inverter in GS4048 to make power and push it back into the grid much like a micro inverter. The GS units will push power into the grid at the full inverter capability. Thus you could have 33 amps of AC-coupled solar flowing backward through the inverter and 33 amps of DC-coupled battery/solar flowing backward through the inverter. This is the more likely scenario of why the AC-couple size is limited.
 
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