How do you calculate the necessary panel box size?
Grid-connected solar photovoltaic (PV) systems generate electricity, supply it to the panel box, and then either send it to the house or the grid. The utility service wires that carry electricity into the meter also carry the excess electricity generated by the PV system back to the utility grid. Typically, both the busbars in the panel and the main breaker in the panel box are rated in amperes according to the service subscription (usually 100 or 200 A for a residential service). There are other breakers in the panel that are rated in amperes based on the household devices they are protecting. The sum of the ampere ratings on all these breakers (with the exception of the main breaker) cannot exceed 100% of the busbar rating.
If a customer decides to go solar and would like to connect the output of their inverter onto a breaker on the load side of his panel, there are few NEC restrictions that need to be adhered to regarding the size of the panel box. The main restriction being that ampere rating of the PV breaker cannot be more than the difference of 120% of the busbar rating and the main breaker rating, according to NEC Section 705.12 (D) (3b).
For example, a panel box with a 200 A main breaker and a 200 A bus bar can only allow an inverter breaker of 40 A (since 120% of 200 = 240 A). In this case, based on the requirement of the ampere rating on the PV breaker, the consumer can go for derating the main breaker to maybe 150 A if their other loads don’t consume more than 150 A. This will allow up to a 40 A + 50 A = 90 A breaker for the inverter. The 120% rule is thus not violated as 90+150 = 240 A.
It is always advisable to get the process of connection done by a professional and the final single line diagram needs to be approved by the local AHJ (Authority Having Jurisdiction).