The Jargon File (version 4.4.7, 29 Dec 2003):

power hit
 n.

    A spike or drop-out in the electricity supplying your machine; a power 
    glitch. These can cause crashes and even permanent damage to your machine
    (s).


The Free On-line Dictionary of Computing (30 December 2018):

power hit

   (Or "power glitch") A sudden increase (spike) or decrease
   (drop-out) in the mains electricity supply.  These can cause
   crashes and even permanent damage to computers.  Computers and
   other electronic equipment should really include some kind of
   over-voltage protection in its mains input to prevent such
   damamge.

   [Jargon File]