I have read the document "AC-coupling and the Factor 1.0 rule" carefully and I can understand the potential problem posed by AC-coupling but I have difficulty understanding how the "Factor 1.0" rule helps in this situation. This rule is simple: “The max PV power must be equal or less than the VA rating of the inverter/charger”. But I do not understand why this rule fix the problem described in section 2.2 “example and background” of the above document.

Let's assume that our installation has a MultiPlus II 5000VA connected to a battery and an AC coupled Fronius Primo 3.6 kW. Now let's apply the problem described in section 2.2 to this configuration. Let’s assume that on a sunny day you need to draw 4000 W from your system: 3.6 kW comes directly from the Fronius and 400 W from the battery through the MP2. Now let's assume that the battery is full and the load is suddenly cut off. The Fronius still delivers full power and the question is how fast the system regulates itself and where the power goes in the meantime.

I can understand that shutting down the Fronius takes time because it involves communication between the two systems: the MP2 has to change the frequency in response to the overload situation and the Fronius has to interpret it before shutting down. However, I find it hard to understand that the MP2 cannot cope with the overload situation because it is internal to the device and the electronics are supposed to react very quickly.

But suppose the MP2 is not fast enough to handle this situation properly, what does the Factor 1.0 rule change? There is no explanation in section 2.2 (or elsewhere in the document) as to why the PV power must be lower than the inverter power and how it fixes the problem.

I would appreciate if someone has an explanation as to why this rule solves the problem and why this factor needs to be 1.0 instead of 0.5 or 2.0?