I find many either use just the watts (P) button or the volts (U) button . Most toppers like Kanger Pro Tanks (especially version I and II ) , Iclears , Vivi's, T-2's , MT3's etc... work great between 8 and 10 watts .
Whichever button you press last is where it will stay so if you press the (U) button last before hitting it you got volts if you hit the (P) button last you got watts . Imo i never saw any noticeable differences between the two except with a limited # of toppers.