As a sidenote, does anyone know whether something like an "optimal" Qi charging wattage exist, as in highest efficiency? Power lost due to resistance increases

As a sidenote, does anyone know whether something like an "optimal" Qi charging wattage exist, as in highest efficiency? Power lost due to resistance increases with current squared. Therefore I would imagine charging <5W would be best for high efficiency. However, Qi is based on quickly alternating magnetic fields. Does the circuitry involved make Qi charging inefficient once the power gets low enough? Or is Qi charging at 0.1W preferable over 2.5W?