text: The longer a computer stays on, the longer it builds up dust inside, which can reduce cooling efficiency of the cooling fans. Heat buildup can destroy the electronic components, especially memory and the CPU. Cleaning the inside by vacuuming it out regularly will help.\n\nThe disk, assuming it doesn't do an auto powerdown, will have a shortened lifespan.\n\nMost modern monitors do go into sleep mode, so that shouldn't matter.\n\nStaying on all the time does, to a very small degree, age the circuitry.\n\nThere is a power savings in turning them off.\n\nI see little advantage to leaving it up 24/7, other than not waiting for bootup and shutdown.\n\nIf you have it connected to a surge suppressor strip, then turning off the strip, as well, provides some protection from lightening strikes. The strip itself can't handle a major voltage spike, like you will see from lightening. Of course, a modem would be an additional point of voltage spike with lightening, one the powerstrip won't help.\n\nThere will be a small amount of power going through part of the system when it's powered down, but still has power to the plug, but most of the circuitry will not be live.\n\nDisadvantages exist if you are connected to the internet 24/7. You would need a good firewall and virus protection software.