Today I signed up with Iberdrola electricity. Extremely easy, we have a lovely agent on the island called Juan Jose he speaks a little English but if you have any problems understanding him, please call Colin and he will assist.
Juan Joses number is 674344467 & Colin's number is 633237191.
They do set monthly payments on the day of the month that suits you, this is revued every 6 months and their tarifs has been capped so no increasing bills.
To change over you need your last bill, NIE and bank details.