It is leaving your laptop in sleep mode that consumes energy, not whether or not it is plugged in

I blogged before about how leaving your laptop charger attached to your laptop can cause it to consume a small amount of electricity if the laptop is in sleep mode as distinct from being either switched off or in hibernate mode.

When I was thinking again about this I realised that this is probably caused by the fact that the laptop's battery needed to be constantly topped up. Therefore I decided to leave the laptop unplugged in sleep mode overnight. When I plugged in charger to the laptop the following morning it started to consume about 24 watts of energy until the battery was back to full power again and then it went back down to the 1-2 watts of trickle power again.

As a result I had not saved any power overall by leaving the charger unplugged overnight.