Abstract The effects of current mismatch and shading on the power output of single photovoltaic (PV) modules are well analyzed, but only few investigations address mismatch losses at a PV system level that also limit the annual energy yield. The simple question, what happens if PV strings with different numbers of modules are connected in parallel, has not yet been discussed in detail. In case of strings with unequal module count, the system builder must decide whether to use inverters with multiple maximum power point (MPP) trackers, module-power optimizers, or to shorten all strings for balancing the system. Our findings from this study open a new option. The numerical modeling of PV systems with strings of different length in parallel to several others which have an equal module count renders mismatch losses below 1% for most system configurations. For configurations where one string is one module shorter than the others, the mismatch losses fall below 0.5%. Therefore strings with unequal length may favorably connect to a cost-effective single-MPP inverter without causing significant energy yield losses. Moreover, typical thin film PV modules are less sensitive to mismatch than crystalline silicon based ones.