The title says it: plugged into the wall (120vac), but disconnected from the MacBook Air, how much power is consumed by the power supply?
I have measured the consumed energy of an Apple MagSafe 2 60 W Power Adapter over a period of three weeks while idle.
The energy consumption drops to 0.00001 kWh per day after less than 30 minutes since the last usage.
I am confident that the MagSafe 2 45 W Power Adapter will behave the same.
Note: Other power supplies I have do consume a measurable amount energy with no devices attached. So Apple seems to have done a really great job building these power supplies.