The general rule of thumb is that a utility service panel's kVA is the upper limit of the kW-AC rating of your solar system.
To calculate the kVA for a three-phase main utility service panel, you will first need to find the voltage (V) and amperage (A) of your panel (usually located on a nameplate sticker).
Once you locate the V and A of your panel, the formula for kVA is as follows:
kVA = V x A x sqrt(3) / 1000.
For example, if you have a 2500A and 480V service panel, the kVA = 2500A * 480V * sqrt(3) / 1000 = 2078kVA.
Thus, the max kW-AC rating for your solar system is 2078kW-AC. Keep in mind, though, that your solar system's kW-AC rating is bounded by the lowest of either your main service panel's kVA or your transformer's kVA. So if your main panel's kVA is 2078kVA but your transformer's kVA is 2000kVA, then your solar system's max size if 2000kW-AC.