Dual inverters

Arthur22

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Joined
Jan 17, 2022
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Hi, new member, first post/question.

I have an existing 4kWp solar array, which was fitted and signed off by an MCS accredited company, I intend to add to it an additional 3 kWp of panels (DIY).

There are many questions and variations as to what I could/should do. My existing inverter (a SolarEdge single phase HD-Wave model) won't talk (as far as I am aware) to a non SolarEdge inverter.

The physical/electrical installation is something I can do without any issues or concerns, other than with regard to safety - not of mine, but potentially the safety of the network operatives.

At present my setup is fully on-grid - I intend to add battery storage at some point (hopefully soon) which will allow for that to change, adding potentially a grid isolation relay/contactor, or whatever is built into the battery/inverter for the same purpose.

I have various options, but I don't have the knowledge to know which is best, hence my post.

I could get an additional SolarEdge inverter for the new panels, connect it via cat5 to the existing one, and the existing one will sort everything out - problem solved.

Or I can use a third party "dumb" inverter - with islanding protection - and connect it to a separate point on the mains circuitry (which would be very convenient).

My questions: Would two different inverters that don't "talk" to each other both shut down in the event of a grid failure, or would they fool each other that each of them is "the grid" - or are they smarter than that?

Secondly, I have my eye on a GivEnergy battery, but that needs to be connected to one of their inverters for warranty purposes, if nothing else.

Are three different inverters on the same mains feed likely to play nicely together? I'm certainly less confident on this point, though I have doubts about the others too! Thanks for any input.

Arthur.
 

Hedges

I See Electromagnetic Fields!
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Mar 28, 2020
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10,976
Two grid-tied inverters, UL-1741 if US market, can both be connected to grid-connected AC panel and things work fine. They export when grid is up, shut down if grid goes down.

There are NEC rules on how much current grid-tie inverters can backfeed into your panel, depending on panel bus bar current rating and main breaker rating. Adding an inverter could exceed that. There are also rules on which slots of the panel get breakers for GT PV inverters.

Ordinarily, I would look into adding panels to an existing inverter, "overpaneling" it. Even if wattage ratings of panels exceed inverter rating, there are ways to use multiple PV panel orientations to get more hours of operation, more kWh, without hitting maximum kW rating.
But yours is SolarEdge. It probably has optimizers per panel. If you can get more optimizers, you may be able to add more panels in series. I think the limit is 24 solar-edge optimizers. However, there is the maximum kW rating you may run into. and while the optimizers support multiple angles to some extent, there is a limit on difference in current (due to angle) they can correct for.

If you go to a battery backup system, then inverters playing together is more important.
 

Arthur22

New Member
Joined
Jan 17, 2022
Messages
2
Two grid-tied inverters, UL-1741 if US market, can both be connected to grid-connected AC panel and things work fine. They export when grid is up, shut down if grid goes down.

There are NEC rules on how much current grid-tie inverters can backfeed into your panel, depending on panel bus bar current rating and main breaker rating. Adding an inverter could exceed that. There are also rules on which slots of the panel get breakers for GT PV inverters.

Ordinarily, I would look into adding panels to an existing inverter, "overpaneling" it. Even if wattage ratings of panels exceed inverter rating, there are ways to use multiple PV panel orientations to get more hours of operation, more kWh, without hitting maximum kW rating.
But yours is SolarEdge. It probably has optimizers per panel. If you can get more optimizers, you may be able to add more panels in series. I think the limit is 24 solar-edge optimizers. However, there is the maximum kW rating you may run into. and while the optimizers support multiple angles to some extent, there is a limit on difference in current (due to angle) they can correct for.

If you go to a battery backup system, then inverters playing together is more important.
Thanks very much for your input - I omitted to mention that I'm in the UK, for which I apologise.

My existing inverter is all but maxxed out with the panels I have on it at present, so it makes sense to run a second inverter to make use of the full capacity of the additional panels.

My export limit is set at 5kW, so I'm curious as to what the SolarEdge inverter will do if it were to see power being fed in and exported from elsewhere? As it has a current transformer adjacent to the utility's meter, I'd assume that it may well ramp down it's own export so as to not go over the 5kW limit? That would be very handy if so.

The installers of the SolarEdge system were keen to supply the LG high voltage battery that SolarEdge recommend, but that's 2 to 3 times the price of the GivEnergy one, and 2kWh lower capacity too - plus safer newer technology lithium cells as a bonus in the GivEnergy battery.

The SolarEdge guys said that their inverter could use pretty much any make of battery, and the GivEnergy guys said that their inverter would happily work fine alongside anyone else's - I'm going to have to tie both companies down to confirm that before I make a purchase.

Never a dull moment!
 

Hedges

I See Electromagnetic Fields!
Joined
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Messages
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I think SolarEdge is a grid-tie PV inverter, and their other product StorEdge is a grid-tie PV inverter that also connects to a battery. So buying StorEdge in the first place would have been the better way to go.


That also says "StorEdge single phase inverter connects to the third-party inverter's AC output. The StorEdge inverter charges the battery using the solar power produced by the third-party inverter."
However, that doesn't mean they play nice together. When AC coupling a separate GT PV inverter with a battery inverter, it is best to have a mechanism to adjust AC production to match loads plus battery charging requirements. Otherwise, GT PV inverter switches off, then back on at full power, and battery cycles up and down. The adjusting mechanism can be varying AC frequency (slightly higher frequency being a request for less power), or separate communications channel (usually within a single brand, although SunSpec standard could open that up.) This is when grid is down, and lets system run powering some loads isolated from grid by a relay.

I don't know if SolarEdge plays nice, but replacing inverter with StorEdge is one approach.

Yes, current transformers measuring net current from/to grid after loads (or other GT PV inverters) ought to let SolarEdge reduce its output on account of other production, to remain within 5kW export limit.

Perhaps StorEdge as an additional inverter can do export limit to 5kW by storing excess in battery (while grid tied), and supplying that power at other times. Separate objective compared to battery backup during grid outages. This is something Tesla Powerwall does. Or SMA Sunny Boy Storage.

Do you have multiple roof orientations to work with? Or would additional panels be same orientation as others?
If you now have 16 panels all facing one way, and you went to 24 panels with 12 on one roof face and 12 on another, that 50% increase might still fit within inverter's capability. It depends on difference in angle and the per-panel optimizer's capability. If it wasn't SolarEdge, just some other inverter, I might wire the panels 12s2p into a single PV input of inverter.

The extra hours of production with multiple orientations helps fit within export limitations as well.
PV panels are cheap, about $0.025/kWh (amortized over 10 years, but lifespan should be several times that.) Just racks to install them adds a significant fraction of the cost.
Name brand commercial batteries cost $0.50/kWh (by the end of cycle life) so using them to shave peaks and move time of use/consumption around is a money-losing proposition.
Batteries only worthwhile to power critical loads during power failures.
Extra PV panels, to make power morning/evening or to make power during the day if you're a net consumer can be cost effective. Even if export limit means production wasted when you're not consuming.
 
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