Electric radiator is 100% efficient.

If your central heating works by heating coils with electricity then it’s 100% efficient as well. If you heat all the rooms to the same temperature it makes no difference which one you use. If you use radiators but leave some rooms cooler than others then that’s going to cost less money.

If your central heating works by burning oil / gas then that’s probably going to produce heat for cheaper than electric radiators are. Same applies if it works by heat-pump. Those are 200 - 300% efficient.