Depends on the charger but either effectively zero or considerably less.

People get pissy about it, but think of electricity like water. Having a longer pipe is a negligible amount of water if the faucet is still off. And the faucet can only turn on if your device completes the circuit by being plugged in (and doing the appropriate handshakes)

That said, some chargers will consume a negligible amount of electricity to actively listen for a device. Think of it like the water in your toilet. Every so often enough evaporates or leaks that you hear it run a bit to refill. But mostly it is nothing until you flush.

They do consume a tiny little bit. I have a Measurement thingy that yoj plug between your outlet and whatever is plugged in which is accurate to 0.1 W. I tryed 3 chargers, one shows 0.1 W, the other two show 0.0 . I still know they consume a tiny bit, but less than 0.1 W is almost nothing. 0.1 W would come out to a consumtion of 0.876 kWh over a year, wich costs me 0.30 €.

A good rule of thumb is that the energy needs to go somewhere. So if the adapter was drawing a significant amount of power, it would get warm to the touch.

My office when everything is off consumes 7w total.

That's 2 high end computers, 3 monitors in sleep mode, 2 phone chargers a laptop charger.

7w is more than tolerable for me

https://en.m.wikipedia.org/wiki/Standby_power

You can always get a Kill-a-watt (or similar if those aren't available for the EU) to see how much power something uses in standby

I remember there being special power strips you could get to detect and stop phantom loads like this. But according to that article, there are now regulations to keep this power draw low, so it's probably not a major problem with modern devices.