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For small scale roof-top systems, there is a trend towards module-integrated electronics. Module integrated ac inverters (ac modules) connect each photovoltaic (PV) module separately to the single-phase grid. They feature not only increased yield due to module-level maximum power point (MPP) tracking, but also further advantages such as reduced installation cost. This work investigates the concept of subpanel-level MPP tracking for ac modules, which allows us to increase yield thanks to reduced mismatching losses. Topology concepts to realize such a converter are systematically investigated and categorized. A topology comparison identifies two promising system concepts: first a single-stage converter with a three-port power balancer and second a two-stage topology with three paralleled dc–dc converters and a pulse width modulation full bridge. The later features the advantage of a small power-decoupling capacitor and is therefore further investigated. A model-based optimization of the investigated multi-input ac module is performed, applying high performing Gallium Nitride (GaN) devices and nanocrystalline core materials to increase efficiency. The built prototype confirms the accuracy of the model-based optimization. The performed efficiency study reveals an achievable efficiency of \eta \text{EU} = 94.5%. In order to compete with single-input ac modules, achieving typically an efficiency of 95.5%, the efficiency of multi-input ac modules must improve beyond the level achieved with the investigated two-stage ac-module topology. Given this result, the alternative system concept with a three-port power balancer in combination with a single-stage converter seems to be more promising, as it is conceptually similar to the high efficient single-input ac modules and may achieve the same high efficiency. [ABSTRACT FROM AUTHOR]