Average power consumed by Digital Set Top box is around 15 Watt to 50 Watt depending on the service provider and type of display i.e. SD or HD, or with and without digital video recording (DVR)
Even if, the Set Top box is in stand by mode, it consumes the power. So, have a habit of switch off the main of Set Top Box. Normally it consumes as much as power when in switch on mode. (Refer to the reference link)
You can see the difference between power consumption when Digital Set Top box is ON and OFF.
Conclusion from the reference:
1. Set-Top Box with DVR: During recording it consumes more power than during off recording conditions.
2. Set-Top Box without DVR: There is little difference in power consumption during On & Off conditions.