ESS Sizing?

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LATTC Student

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I have read this best practices recommendation from a specific vendor we deployed here onsite:

"To maintain stability, no greater than 150% of the [ESS] continuous power capacity (in IQ PV capacity) can be wired into [the energy system]."

I have a 7 (@1.21Amp ea) microinverter system (240VAC*1.21A*7=2.017kWac)
The battery power capacity is spec'ed at 1.28kWac (1.28kWac*1.5=1.920kWac)
So 2.017>1.920 = violation

In the manuf. chart, they make it simple and do show 6 (@1.21A) inverters max with that single battery unit. To comply, I will temporarily disconnect one inverter/module combo from the string until the second 1.28kWac battery arrives, but what stability are they referring to specifically? I am assuming this is a general design recommendation for PV/ESS systems and not manuf specific.
 
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jaggedben

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Northern California
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Solar and Energy Storage Installer
The max allowed PV varies by manufacturer. But the basic concept is the same for all AC coupled systems, which is that if the PV power output is more than the battery can charge at, and any load suddenly drops, you may have a situation where there's PV current with nowhere to go. The system has to respond to that to avoid something getting damaged, and the more inbalance there is, the more difficult that is. Hence the need for limits.
 
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LATTC Student

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jaggedben said:
The max allowed PV varies by manufacturer. But the basic concept is the same for all AC coupled systems, which is that if the PV power output is more than the battery can charge at, and any load suddenly drops, you may have a situation where there's PV current with nowhere to go. The system has to respond to that to avoid something getting damaged, and the more inbalance there is, the more difficult that is. Hence the need for limits.
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This is only for grid-tied whole home (or partial) backups correct? If the design is self-consumption only (power down if grid fails), then the PV array can be oversubscribed (over 150%) in relation to the ESS, no?
 
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LATTC Student

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Ah ok, very cool so limit not implemented when grid is up. So I can have a 200%, or more, ratio of PV/ESS (oversubscribed to charge batteries during PV production) that is grid tied. This is all for self-consumption only. No charging batteries from the grid, no export of either battery or PV to Grid. Thanks again JaggedBen
 
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