It depends on the appliance. In general, "dumb" appliances such as kettles, toasters, heaters etc. have a power switch which internally is-inline with the power cord. So when switched off, these appliances consume zero power. Newer appliances with fancy displays, e.g. microwaves, some washing machines, cookers(kitchen ranges) and such require a little power to keep the display working. The greatest offenders however are set top boxes, (satellite, terrestrial), sound systems, TVs, audio systems etc. when placed on standby (typically anything which has a red LED when asleep). These can consume up to 30% of the power on standby compared to when fully powered up. TVs however have improved somewhat in their power consumption since the move from CRT to LED. Originally the CRT heater was kept powered up on standby which consumed a relatively large amount of power.
The amount of energy used may be small when appliances are on standby, however if you have lots of gadgets, it can add up.