Probably nothing - most electric heaters are very efficient - much will depend on the efficiency of what you have now.
Go here...
http://forums.moneysavingexpert.com/showthread.html?t=2143673
They are not storage heaters and cool down rapidly when switched off - the advertising appears to be a bit shabby and misleading - they appear to be overly expensive for what they are.
Peccavi, December 2009