Based on the facts we've seen established in the series, we know the Q are energy beings and they are capable of taking a physical form. While in a physical form they are proven vulnerable in that form HOWEVER they have some kind of energy store that lets them protect themselves. When they are injured or drained of that energy they are trapped in whatever form they are in at the time and vulnerable as a creature of that form (Deja Q, True Q, Death Wish, Q And The Grey, Q2).
Is that power source something internal to their being or something external, as in the case of Squire of Porthos? We do not know but it seems to be something internal.
I would hypothesize that the Q powers somehow tap into that 'Space, time and thought are all one' stuff referenced in the sillier Wesley stories, and their powers are only as strong as the extent they can tap into that.