Power-on hours (POH) is the length of time, usually in hours,^[1] that electrical power is applied to a device.

A part of the S.M.A.R.T. attributes (originally known as IntelliSafe, before its introduction to the public domain on 12 May 1995, by the computer hardware and software company Compaq),^[2]

It is used to predict drive failure, supported by manufacturers such as Samsung, Seagate, Toshiba, IBM (Hitachi), Fujitsu, Maxtor, Kingston and Western Digital.^{[citation needed]}

Power-on hours is intended to indicate a remaining lifetime prediction for hard drives and solid state drives, generally, "the total expected life-time of a hard disk is 5 years" ^[3] or 43,800 hours of constant use.^[4]

Once a drive has surpassed the 43,800 hour mark, it may no longer be classed as in "perfect condition".^{[citation needed]}

Google tested over 100,000 consumer grade serial and parallel ATA hard disks, finding evidence that S.M.A.R.T. attributes like POH played a heavy role in device failures.^[5]