Can My Panel Handle Solar?

Almost always. By the NEC 705.12(B)(2) 120% rule, a 200A busbar allows 120% x 200 = 240A of total supply. Subtract a 200A main and 40A of room is left for a backfed solar breaker, which rounds down to a standard 40A breaker (NEC 240.6(A)). That supports a 32A continuous inverter output, roughly 7.7 kW at 240V, which covers a typical residential system. If your main is smaller than the busbar (a derated or subpanel main), the room grows: a 200A bus with a 150A main leaves room for a 90A breaker. This assumes nothing else is already backfeeding the bus: if the panel already has a PV or battery breaker, subtract 125% of that too, since the 120% allowance covers the main plus every backfed source.

It is NEC 705.12(B)(2). Where the utility main and a backfed solar breaker sit at OPPOSITE ends of a busbar that carries loads, the sum of 125% of the source output current and the busbar's overcurrent device (the main) must not exceed 120% of the busbar ampacity. In practice, because the backfed breaker is already sized at 125% of the inverter's continuous output, installers apply it as: max solar breaker = (120% x busbar) minus the main, rounded down to a standard breaker size. The extra 20% headroom is the buffer that keeps the bus from overheating when both sources push current at once.

Because the 120% figure is a ceiling on the total, not a breaker size. A 200A bus with a 200A main leaves 40A of room, and 40A happens to be a standard breaker. But a 200A bus with a 100A main leaves 140A of room, and 140A is not a standard breaker, so it rounds DOWN to 125A per NEC 240.6(A). Rounding up would exceed the 120% ceiling and overload the bus. Rounding down is the conservative, universally taught practice: the breaker is always at or below the room the code allows.

Then the load-side (120%) method is not your path, and that is a real answer, not a failure. If the main already uses the full 120% allowance, or the panel is main-lug-only (no main breaker to bound the busbar), you have two routes: a supply-side (line-side) connection per NEC 705.11, tapping ahead of the main, or a listed power control system (NEC 705.13) that actively limits bus current and can fit a larger PV-plus-battery system without a service upgrade. A main-lug-only panel actually uses the 100% sum rule (705.12(B)), not the 120% rule. Center-fed and split-bus panels are also excluded from the simple 120% form. This tool covers the standard opposite-end 120% configuration.

No, and mixing them up is the common mistake. The matrix gives the max BACKFEED BREAKER. The inverter's continuous output current is the breaker divided by 1.25 (the breaker is 125% of the continuous output). So a 40A backfeed breaker supports up to a 32A continuous inverter, about 7.7 kW at 240V. Size the breaker from this rule; size the inverter (or the number of microinverters) to stay at or under the breaker's supported current.