Leaving devices on "standby" means they constantly use a smallamount of electrical power. This amount is much smaller than whenthe device is actually doing something useful. The purpose of"standby" is to have the device available almost immediately, if sorequired.The problem is that if the device remains in standby for along time, the energy thus used adds up. For instance, the amountof energy used every minute might not be significant, but if youleave the device in standby all night (say, for 12 hours), it canbecome quite significant.