Load plays a major role.

Stumbled across this interesting post. Looks like chargers' efficiencies are very similar. The difference is in the maximum output curve. Most chargers deliver a max output of something close to 3W@20kmh, whereas the Forumslader can deliver 5W@20kmh. The maximum drag of most chargers is 3.5W@20kmh vs 6.3W for the Forumslader.

One perspective is to say the higher output levels add (potentially) unnecessary drag. Maybe. My perspective is that since efficiencies are essentially the same, total drag is the same, whether you have a high vs low output charger. It is only a matter of how drag will be distributed over the day. In my experience, batteries were fully charged within a couple of hours on a typical day, meaning that the charger had to deliver whatever wattage my phone would consume once fully charged, i.e. < 2W, A high power charger is more forgiving wrt power management. And drag wattage is very low anyway, and becomes meaningless when your speed goes down, climbing, and you want all of your few leg-generated watts to propel you up that nasty hill...