In addition to the information you've already been asked for, can you tell us how many storage heaters you have, and, if possible what power each one is (2 kW, 2.55 kW, 3 kW, 3.4 kW or whatever)?Got dual tariffs. 70% of the electricity is used at night with the cheap rate. That's why I concluded it was the storage heaters costing money and that's yearly figure. They're turned off in the summer . So in the winter they're probably using 80% of my electricity.
In an attempt to put things into some perspective ... I don't know if they typically remain 'on' continuously for the 7 cheap-rate hours (they might if run on their 'maximum' setting {if it has variable input settings}, but probably not on lower settings) but, if they did, then, for example, a 3 kW one would use 21 kWh per day - which, at the E& cheap rate I'm currently paying (7.844p per kWh), that would amount to about £1.65 per day - i.e. roughly £50 per storage heater per month.
Kind Regards, John