I posted detailed figures from our use of storage heaters over 10 years in a c. 90sqm 3 bed Goldshield townhouse in case they might be a starting point. You'll have to search for it though because I can't remember where the post is. Also your specific costs will depend on other factors too - e.g. the wattage of your heaters, the insulation of the apartment etc. One way to very roughly estimate the cost might be to find out the kW rating of the heaters, multiply by the number of hours that they charge for overnight on Nightsaver and then multiply by the kWh unit cost for that electricity. That will probably overestimate the costs since they operate on a thermostat when charging so the amount of juice used will depend on environmental conditions. On the other hand it gives you a rough worst case scenario cost for operating them overnight. This does not take account of operating any convection only part of a dual storage/convection heater though. There are some other very detailed threads about storage heaters and electricity usage that might be worth searching for.