Critique this Bluetti plus PV home backup system

[PigBodine]

I currently have a grid tied Enphase 4.6kw system and I'm planning on adding about 3-4kw. We have a reliable grid but would like some backup. AC coupling with hybrid inverter and battery seems like an expensive nightmare for the 0.01% of the time I would need it. Our POCO has no time of use.

The new array with be vertical on deck rails meaning easy access. I'd like to run Enphase micros as the additional cost is minimal and I have plenty of space in the combiner box.

In grid down situation I plan to use a Bluetti AC300 with PV input from a normally inactive branch of a Y-connector from the panel MC4 with the other, normally active, branch to the micro for the panels. When the grid goes down the micro's shut down. Bluetti will then plug into a manual transfer switch powering a critical load panel. I would then power that panel and input the PV wiring from the Y-connectors into the AC300 to charge. I also have a method to dual charge the Bluetti from our EV at about 1kw in this situation.

Any issues with this?

Also, as an aside, I plan to run the voltage step down unit sold by Bluetti (D300S) so that I have max 2.4kw solar input for longer during shaded/cloudy times. It will take a max 550VOC string and convert to the max 150VOC for each MC4 input of the Bluetti. Does that seem reasonable?

 

[PigBodine]

Sorry, maybe to much info above.

Here's the short version. Is there an issue using a Y connector with one branch going to Enphase micros and the other only used for an MPPT charge controller when the micros are down? Panels would be in series. Thanks

 

[ejfluhr]

I don't follow your long description. The short one seems to indicate you intend to "switch" the solar panels from the microinverters to the Bluetti, correct?

Nothing fundamentally wrong with that, but details of the implementation matters. I don't grasp how just Y connectors can achieve that. Do you have a diagram?

 

[M.Damon]

As far as I know, the AC output of AC300 can only power loads via a transfer switch, cannot directly connected to the grid or other inverters.

 

[PigBodine]

I don't follow your long description. The short one seems to indicate you intend to "switch" the solar panels from the microinverters to the Bluetti, correct?

Nothing fundamentally wrong with that, but details of the implementation matters. I don't grasp how just Y connectors can achieve that. Do you have a diagram?

Yes, it is a switch of sorts. The AC300 is only for backup in a grid down situation. I have a transfer switch on the load side of the main panel which powers a critical load panel. The Bluetti will input into the gen inlet of the transfer switch.

The sequence goes like this:

Grid goes down, micro's anti-island and shut down. Turn off micro breakers in combiner box to avoid firing micros back up until AC300 is off if grid comes back online. Switch manual transfer over and turn on AC300 and plug in to the gen inlet. Take the normally unused MC4's from the branch connectors and plug into the MPPT's of the AC300 (and B300).




Several panels will be in series for each string to optimize max voltage input of the various MPPT inputs (possible of 4 total in each AC300+B300 combo!)

Thinking about it, this only issue would be the series panels causing some abnormal behavior of the micros as usually each micro would only "see" one panel. Although given the higher resistance of the wiring between the panels, each panel should only feed its own micro. Alternatively, I guess I could hook up the series string each time the grid goes down. Thanks for the help