Inverter for Backup, play nicely with M215's

[Wolfpv]

I have an older 10KW grid-tied system with 48 Enphase M215 micro-inverters. I want to set up a relatively inexpensive battery backup with two 48V x 100A rack batteries. In order to tie to my household wiring I'd need both phases of 240V, and the SW4048 seems to be the only reasonably priced inverter I can find which meets UL1741, and does two-phase. I have a series of questions

• The vendor says it can't work in a grid-tie environment, but the literature says it can. Have you any sage advice?
• The vendor says I absolutely need the battery controller, the battery monitor and the Distribution Panel (and the smart home controller) - the vendor says the system won't work without these. Is that correct?
• All inverter/chargers have separate AC input and AC outputs. What I want is for these to be the same, i.e. AC is wired from the main panel to the inverter's AC input to charge batteries until the grid dies, then AC is produced by the inverter (from the batteries) and fed to the main panel. I isn't practical in my application to isolate critical loads to a sub-panel "downstream" of the inverter. Has anyone accomplished this, perhaps using a manual or auto transfer switch, and if so how have you wired it?
• I'd like to be able to charge batteries and run the house on solar during a power outage. UL1761 seems to allow that, as long as there is adequate demand to absorb the energy produced by the panels/M215 inverters. Is this an unrealistic hope, or would the frequency-shift of the SW4048 accommodate this?
• If I'm totally barking up the wrong tree here, and I can't rely on an automated hybrid inverter to do all that I want, is it reasonable to simply do a battery backup which I manually switch over using a less sophisticated inverter/charger - perhaps like the SungoldPower "6000W DC 48V Split Phase Pure sign wave inverter with charger & UL1741" - and if I do, is there any hope that the UL1741 on this device will allow me to use my solar during an outage.

Sorry for so many questions, but although I'm somewhat techie there's a lot of conflicting info out there. I'm in California and we just had a wildfire start a few hundred yards from our home; we were without power for a few days, and I think it will happen more often in the future. Refrigerators and freezers don't hold the cold too well for more than a few hours!

 

[BentleyJ]

What you are trying to accomplish is completely possible. To Summarize:
A) Start doing some research on AC coupling, Enphase M215 microinverters should be compatible
B) If you wish to do the whole house I would suggest the SW4048 is really undersized. I'm actually doing exactly the same as what you are suggesting with a Conext XW+ 6848.

 

[BentleyJ]

Taking your specific questions as they are bulleted.
1) I'm not familiar with the SW4048, if its compatible with AC Coupling per the literature then it would work BUT as stated its really undersized for the whole house. AC Coupling means the Inverter at minimum has these 2 features. A built in AC transfer switch and is UL1741(SA) compliant.
2) Sounds like the vendor doesn't know what he is talking about. With AC Coupling you leave the Enphase system as is and you don't need a battery charge controller because there is direct connection between the solar panels and the battery. Depending on the battery you probably also do not need the battery monitor either but it does have some nice features. With a single inverter you don't need their particular power distribution panel but some electrical work that includes junction boxes to intercept the incoming power and direct it to the inverter then back to your panel is necessary.
3) Yes, AC Input and AC output are separated. It would be so very nice if there were an AC Coupled system that was bi-directional so all you had to do is install only one 2 pole breaker in your main panel. The older generation Tesla Power Walls I believe did have this single point connection architecture, not sure if that is true any more. 3A) Yes, an automatic or manual transfer switch can be used to intercept the power coming from the utility meter and one of the TS outputs can be used to feed the Inverter and the other TS output can be used to feed the lugs on the main panel. The AC output from the inverter is then connected to a 2 pole breaker in the main panel. In fact, this is exactly how I installed my whole house backup using a Reliance manual transfer switch that consists of 2 interlocked Eaton CH type breakers. One breaker is a 100A the other is a 60A. Under normal conditions the system operates AC Coupled with the main panel being energized through the inverter in Pass-Thru mode. When power goes out it starts inverting in 8ms, we barely notice the lights flicker.
4) That's the beauty of AC Coupling, in a power outage the grid tie solar still works to power the house. Excess AC is used to charge batteries and when the batteries are full the frequency shift will cause the Enphase microinverters to shut down at 60.5Hz. With no solar input the batteries take 100% of the load and begin to discharge. Once the battery voltage falls, the frequency is shifted back to 60.0Hz and the Enphase microinverters will start up. (there is usually a 5 min sync delay)
5) I would strongly urge you to stay away from high frequency, value priced inverters if you what to backup the whole house. Stick with Schneider, Outback or SMA.
6) You mentioned Calif., I live in So. Cal. and the electric rates are nothing short of robbery. The utilities are going to Time of Use rates that are in the 50 to 60 cent per kWh range. I would recommend getting an inverter that has Peak Load Shaving and Self Consumption features. This is another reason to go with Schneider, Outback or SMA.
Let me know if you have more questions.

 

[Wolfpv]

Thank you all for the helpful replies. I realized, after posting this, that though it would be nice to avoid a "critical load" sub-panel, I can't energize the main panel without potentially endangering the utility worker's safety. Unfortunately all my solar and main panel distribution leads to my separate garage, and all the critical loads are off a sub-panel in the house, with no practical way to re-run the wiring between the two (buried in inaccessible conduit runs when I built the house 30 years ago). The house sub-panel is fed by a 100A 240V feed, and I don't really want to size my backup system that big.

I'm thinking that a fully manual backup system which depends upon my switching off the main breaker (to protect the workers) and then connecting the backup inverter's output to the main bus via another switch or relay would work, and (if using an AC coupled inverter) allow the use of the solar panels during the outage. The only disadvantage would be that it would only work if I'm home.... but I'm retired and usually at home! I would, as usual, welcome any and all comments, even if they are to note that I'm barking up the wrong tree.

Incidentally, regarding the comments about the cost of power.... mox nix to me. I installed these 48 panels years ago, and due to the time-of-use policies here - and the local alternate utility option in my area, I've been paying nothing for power, including putting over 100K miles on my Tesla, and I receive a nice check at the end of each year for about $1000 for my excess power! I did the solar install myself, which was easy except for a difficult building authority which ran me through the ringer (I submitted over 100 pages of documents including structural and electrical calcs, equipment specs, schematics, pictorials, overhead view mockups, etc).

 

[BentleyJ]

You Said. "I can't energize the main panel without potentially endangering the utility worker's safety".
I believe you may have missed one of the very important details contained in my above reply. An AC Coupled inverter that is UL1741SA compliant MUST have an internal transfer switch that isolates your home from the grid when the power goes off. So if you want the convenience of having an automated system rather than manual, all you need is the proper inverter and there is NO safety issue.

 

[Wolfpv]

BentleyJ,

Thanks for the note - Such a transfer switch would necessitate a sub-panel, which I was trying to avoid... but now realize it's unavoidable. I simply can't use my backup system to energize my main panel unless I shut off the main house breaker and sever the connection to the grid. That's why, since a critical load sub-panel is not possible in my situation, that a purely manual system may be the best (and perhaps least expensive) system. In that scenario I would manually shut off the main breaker, then re-route the backup output to the main bus bar (through an appropriate breaker). That also would give me the time/opportunity to shut down any large loads before switching over to the backup. It's not ideal, but it's simple, fairly cheap, and should accomplish the task. As usual I welcome your shooting holes in my theories. I do tend to fall in love with my theories!

 

[BentleyJ]

You do have a good point regarding a manual system giving you the time to shut off non critical loads before switching on the inverter.

 

[Ampster]

I would check the M215s to see which version of UL1741 they are. Generic 1741 will not modulate but can AC couple in an on off way versus 1741SA. The difference might affect of hard your hybrid inverter has to work when the grid is down.

 

[BentleyJ]

I would check the M215s to see which version of UL1741 they are. Generic 1741 will not modulate but can AC couple in an on off way versus 1741SA. The difference might affect of hard your hybrid inverter has to work when the grid is down.

I have 11 M250's AC Coupled with a Conext XW+ 6848, they do not modulate, On or Off. Since the M215 is an even earlier version its almost certain to be On or Off only.

 

[Wolfpv]

Agree, on or off...but for this simple, manual backup system that should be just fine

 

[Ampster]

The least expensive manual transfer switch would be an interlock on your main panel.

 

[Ampster]

on or off...but for this simple, manual backup system that should be just fine

I would however research hybrid inverters and find ones that actually have a good AC coupling algorithm. I do not believe UL1741SA in the hybrid grid forming inverter will assure you that it will work well. Some are limited in the GT inverter kW capacity that they can AC couple with. That may also be dependent on battery capacity in the hybrid as well.

 

[Shimmy]

Similar situation (although with a 200A service). I'm going for a XW-Pro 6848, rated 60A. While I resisted it initially, I am going to move my "problem" loads-- the dryer and EV charger-- to a "house" panel, with a manual mechanism to tie them into the critical panel. The whole house will be supported by the critical panel. My normal challenges are easily addressed by the "overload" capacity of the unit, and with the microinverters downstream I have plenty of capacity with daylight.

I am doing a breaker interlock 150A bypass for my system, but that is just because I want an external bypass mechanism.

 

[Wolfpv]

Just to close out my part of this thread...
I received and installed the SW4048 inverter and two EG4 Lifeline 5kwh batteries. The plan, as noted, was to manually switch the inverter input and output during a power outage, making my main panel into the "Critical Load" panel - but first to manually shut off the main breaker to take the house off grid. I hoped that I could still use my 46 solar panels with Enphase M215 inverters during an outage.
Bottom line: it works perfectly, but it took a bit of doing. The only challenge was that the M215's initially didn't want to recognize the "fake" grid - they were throwing over-voltage alarms. It must have been due to spikes because I was reading constant voltage within range. Nevertheless, the nice Enphase lady in India agreed to switch my inverter profile to "SAM-60-120-1304 ver. 2" which is the most variation-tolerant profile (you can't do it yourself without a "token"). That did the trick, and all my inverters came online (albeit a bit slowly). Anyway, this worked without any of Schneider's supposedly required accessories, e.g. DC Switchgear, AC Switchgear, Battery Monitor, SCP, ad infinitum. I did order, but haven't yet received, the Insight Home device which will allow both detailed setting and control of the inverter and online monitoring.

I've had two utility power outages totaling about 8 hours in the two days since this was installed (good timing!) and though the manual changeover does take about 30 seconds I've otherwise had all the power I needed. 10KWH should cover nighttime needs easily, and I can recharge from solar during daylight. The total cost of the system was just about $5K, including two EG4 48V 5kwh batteries, the inverter, the Insight home controller, and the A/B switch. I happened to have the necessary battery cables and terminals as well as the AC wires in my collection of stuff. I'd be happy to answer any questions, of course.

 

[Shimmy]

Looks good. What is the selector switch-- 6-pole? Seems like a great way to set up a test system.

 

[BentleyJ]

Good job, its really nice to have back up power in a grid down situation. Funny how timing works out sometimes. We had a scheduled power outage for a neighborhood transformer change out the very next day after our back up system install was finished and turned on.

 

[Wolfpv]

The manual switch is from Amazon:

Baomain Universal Rotary Changeover Switch LW28-32/D303.3D with Master Switch Exterior Box 660V 32A 12 Position 3 Phase.​

​

 

[GVSolar]

NOTE: Essential that you set your AC charging specs to the proper lfp profile. SW default profile may be too high (90 amps on the 24v model) and pre-set for lead acid voltages. You will need SCP, insight or Gateway to do so. Suggest you disable charging until you have verified the proper settings... No need to whack your new batteries. Best.

 

[Wolfpv]

GVSolar, thanks for the heads-up. My Insight home arrives tomorrow so I'll have control over all parameters. Luckily the 45 amp limit on the SW4048, split between 2 batteries, won't over-tax them. Generally LiFePo4 batteries with a decent BMS can handle lead-acid charging specs as long as equalizing is shut off. Anyway, thanks again, and all suggestions are welcome.

 

[400bird]

Nice set up!

I'm a bit confused/concerned by your AC coupling. Maybe I missed something.
You've got 10kW of solar hooked up to 3.4/4kW of inverter.

The DC charger looks like it's limited to 2kw or so (45 amp at 48 volt)

Seems like a recipe for trouble. Normal recommendation is to limit your AC coupled PV to match your DC inverter.

Do you have a way to disconnect 80% of your solar when off grid (breakers)?