I'm not sure how to best compare apples to apples here. In general, does it use more energy to use a separate whole-house dehumidifier or to use the extra-low fan speed "dehumidify" mode on, say, an Infinity system?
I can see how using the central a/c dehum mode could be more efficient on cooling days. It seems that the separate dehumidifier would be better on heating days if humidity was still too high - granted, that's less likely.
What about shoulder-season days where there's no call for heat or a/c? The system is running for ventilation, air filtering, and dehumidification only. In that case, how can I compare the costs of using "dehumidify mode" with a separate dehumidifier?