Is it possible to use RV system to drive Ground Based PV when the grid is down?

SteinVT

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Jul 30, 2021
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I just finished installing an 11.7kw ground based, grid tied system in my field. Works great, love watching the meter run backwards. It uses Enphase 7+ micro inverters. The way I understand it, if the grid goes down, the micro inverters shut down. Seems like that would be very frustrating if we were without power and the sun was shining.

My current project involves add 1.2kw of solar, batteries and converter/inverter hardware to a 1962 Airstream Travel trailer, which I am restoring. Still in the planning stages for the electrics of the trailer project. But I got to wondering, when the grid goes down, could I disconnect the house from the grid and plug in the trailer. If my trailer inverter was 240v, would it then fool the micro inverters into producing power? I couldn't store it but I could use it in the mini-splits to warm the house and cool the freezer. If I only did it when the sun was shining, it still would be much better than nothing. I would have to monitor it closely, as the house could drain the batteries pretty quickly if the sun disappeared. When the sun is out we make 2 to 3 times our use even with the mini splits on.

I also read about the new Enphase 8+, which is supposed to have some of this capability. Don't want to swap out all 32 inverters, but I wonder if it might work to swap out one? - Mark
 

sunshine_eggo

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Typically, yes. You would get best results with an RV inverter capable of AC coupling. An AC coupled inverter can sense the additional grid power and use that surplus power to charge the RV batteries from the grid-tied PV.
 

caki

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Jul 18, 2021
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My understanding is that you need to modulate production to match consumption and that is what the IQ8 does. I think a good analogy would be to imagine the inverters as producing water and keeping it at 240 psi. If there is too much production pressure goes up, too much consumption and goes down and the inverters would shut down in either case. On the grid, there is always a place to draw or send water to. In a microgrid, the tolerance is really small to keep the pressure at 240 psi. That's where a battery bank helps to smooth it out.
Anyway, if you find out I'm in the same situation. Have a 7kw system and finishing a trailer build where I'm adding solar and would love to join both.
 

Hedges

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Mar 28, 2020
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Typically battery inverters that do frequency-shift can support up to their own wattage of GT PV inverters that do frequency-watts. Some support 2x their wattage.
If you have 11.7kW of Enphase, may need to shut off some of them to keep wattage what your RV inverter can handle.
I though Enphase 7+ also had the capability.
You can put an interlocked breaker in your main panel, a generator input. Output of RV system would go to that plug.
Large household loads could be controlled according to battery SoC or voltage, or by light sensor, or timer.
For anything with a thermostat, small relay would be sufficient.
You could use presence of AC on "generator" plug at panel to control when this load-shed mode should be active.

My home grid-backup system has a relatively small battery, and timer settings of automatic setback thermostat will help, but I should add more controls to automate nighttime load shedding. Otherwise, entire house load gets shed at 70% DoD.

I use SMA Sunny Island (heavy, 140# and 120V). There is Schneider (heavy), but I think Outback Skybox is not quite as heavy (110#).
Anybody know what other battery inverters do frequency shift? Maybe a high-frequency split phase unit?
Of course there is Encharge (expensive battery)

Needs to support starting surge. Mini-split may not have as large a surge.

If RV has its own DC coupled PV, that helps recover from over-discharge. My SMA would shut off at 80% DoD and not capture any AC coupled PV. I avoid that with a load-shed relay that disconnects loads but not GT PV inverters. DC coupled could make that simpler, but you lose out on available AC coupled PV until battery SoC high enough, and then have to manually restart. Best to shed loads and keep system operating.
 
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