You need to do the maths...
Find out the total rating of the heater(s) in Kilo Watts (kW) - it should be on a plate somewhere on the heater.
Them multiply this by the number of hours the heaters have power during a week.
The answer will be in Killowatt Hours (kWh) then multiply this by the cost of each kWh unit.
I do not have storage heaters but in the UK they're usually connected to a special low rate economy supply as far as I know...
http://en.wikipedia.org/wiki/Economy_7
Standard cost of a kWh varies around the 7p mark
The cheaper economy 7 cost can be as low as 2.3p per kWh but has recently gone up to double this in some areas.
Peccavi, November 2010
