User blog comment:Leek0421/The strongest Female Fairy Tail wizard/@comment-112.203.179.223-20150512141443/@comment-7418318-20150513070331

True we don't know how the initial summoning is powered... considering how Lucy was completely out of magic to the point of being unable to move her body it's even likely that the energy came from either Aquarius or the CSK himself.

But going by what happens once the King is in the material world and his exchange with Mard Geer we can pretty savely assume that keeping his gate open constantly drains power from the summoner and quite a lot of power too.