Executing killpower is needed to ensure that the system will automatically
reboot when the mains power is restored [1].

The script was taken from Gentoo [2].

[1] http://www.apcupsd.org/manual/manual.html#shutdown-sequence
[2] https://gitweb.gentoo.org/repo/gentoo.git/tree/sys-power/apcupsd/files/apcupsd.powerfail.init