If you are on OH 4.2 and the Item is a Number:<something> then all the persistence actions return the unit. If you are on an earlier version of OH the persistence actions do not return a value with the unit.
So really, the better way have handling that is to upgrade to 4.2 and make sure that AC_Available_Power_W is a Number:Power and for good measure set the unit metadata on that Item to W.
But even on older OH 4.x versions, if you send a number without units as an update, the default unit of the Item is assumed. So if you just sent 25, OH will assume it’s 25 W. So you don’t really need to do anything special in your rule to handle the units at all either way. I still recommend setting the Item’s unit metadata to W instead of relying on the default which, in this case happens to be W but it won’t always be the case that the default unit is what you want.