A small circuit can be added to power adapters to detect if current is being drawn. When there isn't an appliance drawing DC power, the AC power to the transformers windings is disconnected, leaving only a charged capacitor waiting to activate a switching circiut to power the windings once current is on demand again.
Similar to the "energystar" rating icon, a new icon can be used to promote the use of the new adapters. These new adapters would have standardized fittings that can be used with any DC product of the same voltage, ie; a standardized fitting for each voltage amount.