I have 2 integrated heat/air con units, no fires, and even though it heats the whole property plus I have a double height ceiling in the lounge,after 30 mins one system needs to be turned off. because it is too hot . It is very expensive to run (1,500e over 6 months) in comparison to my neighbours electricty bills, but having said that when one adds in the cost of their wood, gas bottles or oil it works out about the same.
The main disadvantage using electricity against other forms of heating though is that it cools very quickly plus a fire always feels more cosy even when it is just the dying embers. Perhaps have just one wall unit in your lounge as a back up on the really cold days..
I have investigated the two tariff system but it isn't practical if the heating needs to be on before 10pm. Which in winter it certainly does.
Take notice Mr Clegg ,surely your wife has told you wintertime in Spain IS cold.