A demand-side dimmable LED lamp operable on a direct current power source that powers a lighting subsystem. The dimming unit selects a power consumption level of the lighting system. Such selection changes the efficacy of the lighting subsystem such that a reduction in power consumption actually results in improved efficacy. The selecting might, for example, select a particular passive network that includes LEDs within the lighting subsystem. Each passive network may have different I-V characteristics, and result in different L-P characteristics, thereby effecting the improved efficacy at lower powers.