So, according to the core rulebook pg 144 "…a standard non-military power supply lasts for 1d5 hours…"
This sounds kinda silly to me. Like, I am gonna wear this power armour and lets hope it's battery wont die in the middle of battle. For the Emperor!!
I understand that, this is the non-military power supply, but acolytes may don't have the necessary resources to outfit a power armour with a military power supply, witch in my guess works like the sororitas power armour.
My suggestion: allow the player wearing the armour to get constant "power readouts" so they know approximately how much time they have left. I would also remove the random factor. Armour is powered for 5 hours assuming light activity (walking, etc.). Combat and strenuous activity reduces this (maybe 1 round uses the equivalent of 10 minutes?) Good- or Exceptional quality armour might extend the battery life.
As you say, it's not worth the risk if you;re taking your life into your hands every time you don the armour.