Does a power cord use energy when it is not pugged into a Mac?
Yes, the charger will use power when plugged in, but much less than when it is actually charging.
Once the mac is charged then unplug the power supply - it serves no purpose leaving it in, but if you forget it is not a huge waste of power.
The term for this is vampire power and if you assume the MacBook charger in this test was equivalent to 3 of the tested chargers combined, it might consume 18 cents of electricity per year in the US.
- https://www.howtogeek.com/231886/tested-should-you-unplug-chargers-when-youre-not-using-them/