There isn't a "low power" mode per se. Your Apple Pencil is either completely motionless and "sleeping" - or is fully active.
Have you enabled Find My for your Pencil? If yes, be aware that your Pencil will consume its limited battery power faster than when Find My has not been enabled for an Apple Pencil Pro.
It is generally recommended that an Apple Pencil is docked with the host iPad whenever it is not being actively used - as this will keep the Pencil's battery charged and ready for use. The Pencil will suffer no ill-effects for being docked for extended periods; the host iPad will optimally maintain the Pencil's battery - and will not over charge the Pencil.