It's a very big question...
Running costs or installation costs or both together and if so over what period of time?
What sort of property - how well insulated and how many rooms and what type of use?
Which country and what electricity supplier - regular metering or reduced off-peak tarriff?
Some information can be found here...
http://www.nef.org.uk/energysaving/storageheating.htm
Good luck...
Peccavi
July 2009