Normally there is no power consumption when the dimmer is off.
The consumption of power by the dimmer depends on what kind of dimmer you have. A PWM dimmer consumps less power then the normal dimmer. The lost of power of a normal dimmer (zero cross dimmer) is only when the dimmer is conducting (between 0° – 180° and 180 – 360°) so the time of conducting and the voltage over the triac and the current flowing in the triac makes the power lost.
If you search the internet you will find some formula’s to calculate the losses