If it's turned off or it is on and not being used, probably not much. But more and more equipment no longer has an off mode and is more of an idle stand-by mode, so current is still always being used by part of the system.
The point is that in electricity delivery, it is normal to have deviations in the voltage as acceptable. Most power supplies have tolerances that can handle these deviations. A dip is something outside of the deviations that most people don't consider as they are only focused on spikes and only use surge suppression devices. Since the only protection from dips is a more expensive battery solution, most people do not bother outside of computer related usages.
Ah, so e.g. something like a device flipping between “powered” and “not powered” states, which then wears out some part that doesn’t expect that state change to happen so often?
no, not quite. a dip is some lower than the accepted deviations from the expected value. if you have an accepted range of +/-10%, but the dip causes the voltage to drop 15%, that's a dip. some places define an outage as being less than 5% of expected value. so somewhere before outage, you have a severe undervolt situation where the equipment struggles. the longer the dip lasts, the worse things can get.
one of the shops that I worked at suffered a catastrophic situation when a nearby construction crew cut one phase of the 3-phase power coming into our facility. the poor transformer died, and took out the some of the more delicate PSUs attached to some very expensive equipment. we were down for weeks recovering from that.
The point is that in electricity delivery, it is normal to have deviations in the voltage as acceptable. Most power supplies have tolerances that can handle these deviations. A dip is something outside of the deviations that most people don't consider as they are only focused on spikes and only use surge suppression devices. Since the only protection from dips is a more expensive battery solution, most people do not bother outside of computer related usages.