How much energy is wasted if I leave my laptop charger on 24 hours without actually charging?

For Now I have taken MacBook Air, As Example!

The vast majority of electronics operate in either an “on” or “stand-by” states when connected to power.

That is, the power circuits and systems in any modern device (except for special exceptions) never shut themselves completely off if power is available. The controller or “brain” of a power supply will idle in a low power state and wait for power demands to increase.

The microchips and “brains” of modern laptops and cellphones operate in a similar fashion. I wouldn’t expect the stand-by power needs of a modern device to be more than 5% of operating power (most are 2% or less) when correctly shutdown.

The amount maintenance power needed for a computer that isn’t shutdown but in an in-between state (system stand-by, sleep, hibernate) depends greatly on how a device is setup but will be more than the shutdown state.

To achieve more power conservation than a standard shutdown would require some sort of external control system would disconnect a device (laptop) after charging and ensure the device is fully shutdown (a human qualifies as an external control system).

This will save a few watts an hour in power supply waste.

Shutting down a device that isn’t in use and keeping the device connected to power is generally the most practical power conservation option, even if the device’s power usage doesn’t go all the way to zero.

As an aside: Desktop computers plugged into the wall and are “off” (not in “sleep” or “stand-by”) have the same kind of 1 – 5 watt per hour waste as a laptop that is “off” and charging perpetually.