I suspect that the method of paying your bill depends on the setup that was in place with the previous owner.  In my case, I enquired about getting the name changed on the account for this house.  I was advised I needed to go to the Comarca in Amposta.  My Spanish was even worse in those days but I gathered that it would require me to have an architect inspect the house to confirm it's suitability for supply (which had been in place for years) and that the privilage would cost me about €800  :o  By a stroke of luck, when we rented a P/O box in the village, I was allocated the same number as the previous occupant so I now have my bills delivered (except for last month's) with the old name on and I just pay at the P/O and keep schtum.  Sometimes you have to bend with the breeze rather than break in the wind.