(Been a while since I started a thread here! ) I recently finished some efficiency testing on the Model S Gen2 chargers (pre-refresh, but VIN > ~20k). Tests were done at 240V single phase. 5 amps, 82.77% 10 amps, 89.53% 15 amps, 89.06% 20 amps, 90.04% 25 amps, 91.03% 30 amps, 90.37% 35 amps, 90.75% 40 amps, 91.75% 45 amps, 90.81% 50 amps, 90.88% 55 amps, 90.55% 60 amps, 90.32% 65 amps, 90.21% 70 amps, 90.55% 75 amps, 90.77% 80 amps, 91.63% Keep in mind that the gen2 chargers are actually three smaller chargers in one, and that dual chargers would be six smaller chargers. So, during the current ramp up different numbers of these are online which explains the dips and jumps at say, for example, 25->30A. The data was collected over several days from two different vehicles with dual gen2 chargers and averaged. The number represents overall system efficiency (actual power into the battery pack while charging vs actual power into the charge port) which would include coolant pumps and other items needed for charging. Tests done at room temperature (~70F/21C) This doesn't take into account losses through the EVSE cabling and such, which will vary depending on design and other factors, but shouldn't be appreciable enough to matter when pitted against charger system efficiency. TLDR: For highest charger efficiency, charge at 40 or 80A.