Re-using grid-tied panels off-grid during extended outage

[agt]

I'm about to install (via contractor, non-diy) 20x Panasonic N330E (panel+ Enphase iQ7+ microinverters), and am pondering how I could make use of them during an extended grid outage (earthquake, zombies, etc. so I guess not too far from the standard prepper scenario).

Grid-tied or hybrid batteries weren't in the cards, so I'm thinking of functionality more along the lines of a Sunny Boy secure outlet, optimized for minimal cost, and able to power for a few hours a day an upright freezer, or trickle-charge an EV at the lowest 720W charge rate, and low-power phone/laptop chargers.

My idea, in the event of an extended outage, would be to get up on the roof;disconnect 4+ panels from their microinverters; run new MC4 cable into a parallel combiner; then to a Victron 100/30 feeding into a 1500W 24V inverter. A pair of small batteries would help buffer demand surges, but my plan would be to draw significant power only during full-sun periods. PVwatts says with 4 panels I should see 600+ watts >4/hours/day for ~300 days/year.

The batteries seem like the weak point here, but my hope is that they'll only come into play when the freezer compressor starts up, or if clouds/birds shade the panels during heavy continuous load (eg EVSE).

Outside of zombie uprisings I figure I'll use this this kit for camping alongside 2 50W 12 V panels I already own, maybe in a 12V configuration with a smaller inverter.

Does this seem reasonable? Crazy?

 

[Hedges]

It appears Enphase offers products to do that:

Solar Islands: Addressing Blackouts and Outages with Grid-Agnostic Solar

Prevent blackouts & power outages - be Always-On whether you are on-grid or off-grid. Install the grid-agnostic Ensemble™ energy management technology. It ensures power production whether you are connect to the grid or not.

enphase.com

Reliable backup power for your home | Enphase Storage

Protect yourself from blackouts with Enphase Solar and Storage. Our battery system utilizes safe, low voltage power to intelligently provide reliable battery backup for your home.

enphase.com

Solar inverter from Enphase to run when the grid crashes or doesn't exist - without batteries

Enphase Energy, in their 4th quarter earnings call, made references to their upcoming ‘Ensemble’ line of solar inverters. Their Chief...

electrek.co

 

[agt]

@Hedges thanks- it's an attractive system, but I can't spare the >$6k for the smallest configuration.
My budget is more like $600...

I'll keep this thread updated with my progress!

 

[newbostonconst]

Your layout looks sound to me....I just wouldn't want to have to unhook the micros....

A GS4048 @ $2,000 and a 48v battery would really work for you and be the cheapest option to be able to keep your array the way it is. I know it is more money but you could add bigger batteries later and run all night. I have 24 IQ 7's and run this way.

Good Luck....I do like the idea and layout you have.

 

[Hedges]

You could run MC cables from the panels, from the micro-inverters, and from combiner, to a box
Then when micros are shut off because grid is down, and with a switch to charge controller shut off, you could open the box and swap pairs of PV panel MC cables between micros and charge controller.

Don't forget fuses or breakers when you parallel four panels. If you can find a charge controller which accepts higher Voc, you could wire panels 2s2p and no fuses needed. But breakers either way and you can shut them off while changing MC cables.

I'm thinking of something similar to move panels between string inverters.
I have 3-pole 600V 30A disconnect switch rated for DC. I could feed up to 3 strings into it.
When I turn off the switch it breaks the strings so no current flowing.
Only with switch off, box can be opened to to access 3 pairs of MC3 cables from panels, 3 pairs from inverter set "A", 3 pairs from inverter set "B"

Purpose would be to feed inverter output to the grid, vs. placing on the output of Sunny Island when off-grid. (Current limit of its internal relay.)
I'll use MC3 because they just have rubber gaskets, no mechanical lock. And connector quality looks better.
Originally I planned to switch AC not DC. Fewer inverters. But I have inverters coming out of my ears now.
There are also communication/configuration differences, so inverter supports UL1741 vs. island mode.
Either way, a relay would interrupt AC to disable inverter wired through Sunny Island when it connects to grid (supports twice as much AC coupled PV off-grid as on.)

 

[Maast]

What you're thinking of doing can be accomplished with whats called "AC coupling". You would need to have a whole house transfer switch going to a split phase hybrid inverter that has a battery bank. When the grid goes down you transfer over to the inverter and since your solar panel inverter "sees" power in the lines it will start producing again and feed power to the hybrid inverter which will then in turn charge the battery bank.

Even though I REALLY dislike the current crowd favorite of the MPP solar inverter (HUGE idle loss of about 70 watts) I believe it can do this and they're under your $600 budget, you'll still need a battery bank though. I know for sure the Magnums, Outbacks, SMA Sunny Island, and Victrons will AC couple.

Edit: This should get you started https://www.backwoodssolar.com/learning-center/ac-coupling-explanation

 

[Hedges]

You would need to have a whole house transfer switch

If manual rather than automatic is OK, a $50 interlock between main breaker and backfeed will do.
That would normally be used with a generator. I connected it to the output of my battery inverter.
Because same panel feeds input to same inverter, it would think power had returned and try to reconnect to the grid. So I manually flip breaker feeding inverter as well.
For OP, it would be the breaker feeding his Enphase he would want to manually flip, so Enphase doesn't try to feed his "24V 1500W Inverter"

Only catch with this setup is it is difficult to tell when grid power is restored, unless you can see your neighbor's lights. Could add a pilot light to line side of main breaker.

"MPP solar inverter (HUGE idle loss of about 70 watts)"
Does it have a sleep mode where it turns off, waking up occasionally to see if any loads are present?

 

[DavidHouse]

I have a very similar question to the OP. We have a large contractor installed micro inverter system which is grid tied. Separately we have a small off grid system (2x100 W panels, 1 kWh lithium iron phosphate storage) for power outages, not connected to the house wiring at all (use an extension cord to the inverter when needed).

The question is whether there would be any issues adding a Y connector at the MC4 connections for a couple of the 300 W panels from the grid tied system, so the micro-inverters could keep running, but I can also tap off DC power for the off grid system. Everything in the off grid system is sized properly to accept that. Any issues?

 

[Hedges]

I have a very similar question to the OP. We have a large contractor installed micro inverter system which is grid tied. Separately we have a small off grid system (2x100 W panels, 1 kWh lithium iron phosphate storage) for power outages, not connected to the house wiring at all (use an extension cord to the inverter when needed).

The question is whether there would be any issues adding a Y connector at the MC4 connections for a couple of the 300 W panels from the grid tied system, so the micro-inverters could keep running, but I can also tap off DC power for the off grid system. Everything in the off grid system is sized properly to accept that. Any issues?

Yes, probably an issue. I assume the micro inverters are not transformer isolated. When operating, the PV panels may have an ohmic connection creating a reference to voltage of the grid wires. You generally can't connect two devices to the same PV wires (though would work for something floating or AC coupled that doesn't impose a voltage reference.)

I would recommend completely isolating both PV leads from the micro inverter. That's why I said extension MC cables. You need to shut off inverters or charge controllers when mating/unmating cables, which is why I plan to put the cable ends inside a disconnect switch. Otherwise you have to remember to shut off each manually.

 

[DavidHouse]

Excellent point, hadn't thought about that. So I can switch a couple panels over manually if there's an extended power outage, but no way to have both systems connected simultaneously. Makes sense.

And it could be set up for a quicker switchover, but each panel needs to have both cables disconnected from the micro-inverters and connected up to the DIY off grid system, which adds to the complexity.

 

[Hedges]

Here's a 2-pole transfer switch for $10.
It is intended for AC, not DC. Can't be relied on to interrupt DC if current is flowing at the time it is switched.
Not rated or listed for DC (for that matter, probably not rated by any recognized body for any purpose.) Caveat Emptor.

2P 63A MTS Dual power Manual transfer switch Circuit breaker MCB 50HZ/60HZ 400~ | eBay

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Four of these would transfer four, 2-wire devices.
You can get 3R rain tight boxes for DIN rail breakers.

If you parallel more than two PV panels (or strings of panels), each string should have its own fuse.

 

[rin67630]

What do you expect from a 350Wh battery?
Half an hour operation?
I'd go for a dumb gasoline emergency power generator.
At least you will be able to run it for more than a few minutes before being out of juice.

 

[Hedges]

Here's a 2-pole transfer switch for $10.
It is intended for AC, not DC. Can't be relied on to interrupt DC if current is flowing at the time it is switched.
Not rated or listed for DC (for that matter, probably not rated by any recognized body for any purpose.) Caveat Emptor.

2P 63A MTS Dual power Manual transfer switch Circuit breaker MCB 50HZ/60HZ 400~ | eBay

Find many great new & used options and get the best deals for 2P 63A MTS Dual power Manual transfer switch Circuit breaker MCB 50HZ/60HZ 400~ at the best online prices at eBay! Free shipping for many products!

www.ebay.com

Four of these would transfer four, 2-wire devices.
You can get 3R rain tight boxes for DIN rail breakers.

If you parallel more than two PV panels (or strings of panels), each string should have its own fuse.

On second thoughts I am not going to even suggest these transfer switches for any purpose.
I just heard a buzzing noise from mine. It was holding off 240 VAC across each pole of the switch in "off" position (because other AC source was out of phase.) Isolated the other source at its own breaker, and buzzing stopped.
It is labeled for 400V but I don't trust it at all. Will replace with a name brand.

 

[agt]

@rin67630 My hope (based on reading around the web, and some EE classes 20+ years ago, but no real-world experience with off-grid solar) is that the batteries will only supplement the charge controller during momentary peak loads, e.g. surge during freezer motor startup, or a brief bird/cloud shadow on otherwise fully-lit panels. While small, they would only need to supply 800-900 additional watts for periods of a few minutes, and 1300-1500W for periods of a few seconds, all within spec. Steady-state load would be handled by the overpaneled charge controller's 600-800W, and any battery charge consumed during peaks should be quickly replenished. Again, this is all theory until I can test, and I welcome feedback from anyone with experience.

Ongoing maintenance should be far less effort than for a generator, with no need to start up every few months to maintain lubrication, or maintain (and rotate) a supply of stabilized fuel.

 

[gnubie]

That's it exactly which is why I am always telling people to have their SCC 'float charge' their lifepo4 batteries at a safe voltage. It allows the charger to always be supporting the load fully or at least partially once the battery is charged.

 

[rin67630]

@rin67630 My hope... is that the batteries will only supplement the charge controller during momentary peak loads, e.g. surge during freezer motor startup, or a brief bird/cloud shadow on otherwise fully-lit panels.

So, where you live, there is no night?
Expect power outages to come together with many other adverse conditions e.g. heavy clouds, broken trees, smoke, ashes that will inhibit your solar installation to run as usual.
At least you should add a circuit to exclude the fridge and every non essential loads during night times from emptying your battery.
Some bottles of salt water in the fridge can store more "cold power" for hours than every costly battery.

 

[Hedges]

@rin67630 batteries will only supplement the charge controller during momentary peak loads, e.g. surge during freezer motor startup, or a brief bird/cloud shadow on otherwise fully-lit panels. While small, they would only need to supply 800-900 additional watts for periods of a few minutes, and 1300-1500W for periods of a few seconds, all within spec. Steady-state load would be handled by the overpaneled charge controller's 600-800W, and any battery charge consumed during peaks should be quickly replenished. Again, this is all theory until I can test, and I welcome feedback from anyone with experience.

It works that way for me.
You need enough PV to supply the loads. Ideally, some way to shed loads when production is lower.
And, you need the batteries to handle any current the charge controller puts in them when they happen to be low. Setting up the charge controller to do that is the trick, if your PV is large. Other guys here say some DC charge controllers paired with same-brand inverters are able to do it.
I have mine working that way because PV is AC coupled, and the battery inverter decides how much DC current to generate from AC. At the moment my PV production is 1C relative to battery, and battery charge is set for 0.2C

I currently just load-shed the entire house at 70% DoD. Plan to add controls to disconnect refrigerators and A/C at night. But my battery is big enough to run them, just cycles more deeply this way. I also plan to double PV. Then I could get by with 10% to 20% of full PV production.